research paper on biogas plant

A critical review of biogas production and usage with legislations framework across the globe

• Published: 16 May 2021
• Volume 19 , pages 3377–3400, ( 2022 )

Cite this article

• S. Abanades 1 ,
• H. Abbaspour 2 ,
• A. Ahmadi 3 ,
• M. A. Ehyaei   ORCID: orcid.org/0000-0002-4721-9427 5 ,
• F. Esmaeilion 6 ,
• M. El Haj Assad 7 ,
• T. Hajilounezhad 8 ,
• D. H. Jamali 9 ,
• A. Hmida 10 ,
• H. A. Ozgoli 11 ,
• S. Safari 12 ,
• M. AlShabi 13 &
• E. H. Bani-Hani 14  

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This review showcases a comprehensive analysis of studies that highlight the different conversion procedures attempted across the globe. The resources of biogas production along with treatment methods are presented. The effect of different governing parameters like feedstock types, pretreatment approaches, process development, and yield to enhance the biogas productivity is highlighted. Biogas applications, for example, in heating, electricity production, and transportation with their global share based on national and international statistics are emphasized. Reviewing the world research progress in the past 10 years shows an increase of ~ 90% in biogas industry (120 GW in 2019 compared to 65 GW in 2010). Europe (e.g., in 2017) contributed to over 70% of the world biogas generation representing 64 TWh. Finally, different regulations that manage the biogas market are presented. Management of biogas market includes the processes of exploration, production, treatment, and environmental impact assessment, till the marketing and safe disposal of wastes associated with biogas handling. A brief overview of some safety rules and proposed policy based on the world regulations is provided. The effect of these regulations and policies on marketing and promoting biogas is highlighted for different countries. The results from such studies show that Europe has the highest promotion rate, while nowadays in China and India the consumption rate is maximum as a result of applying up-to-date policies and procedures.

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Introduction

From the 1980s onward, the striking jump in global energy consumption has been largely driven through fossil energy resources. Generally, oil, coal, natural gas, electricity, nuclear energy, and renewable energies have shared 33, 27, 24, 7, 4, and 4% of total primary energy proportion in the whole world in 2018, respectively. Approximately, 85% of the world's primary energy consumption has been supplied by fossil fuels in 2018 (BP. 2019 ; Ghasemian et al. 2020 ).

The conversion of biomass to energy has been promoting from 65 GW in 2010 to 120 GW in 2019 due to climate change, reasonable energy prices, distributed generation increase, and environmental aspects, in recent years. Wastes with high moisture content are more compatible with conversion by anaerobic digestion, landfill, and digestion technologies. The global amount of biogas plant capacity was about 19.5 GW at the end of 2019. Organic wastes are the most common feedstocks to produce biogas from wastes, including domestic wastes (food, fruits, and vegetables) or public moist wastes (cafes and restaurants, daily markets, and companies’ biological wastes), due to significant moisture content and high degradability. These input materials are classified as OFMSW, which represents the organic fraction of municipal solid waste (Antoine Beylot et al. 2018 ; A. Luca C.R. 2015 ).

Biogas is inherently renewable, on the contrary to fossil fuels, because it is generated from biomass, and this source is practically a reserve of the solar energy via photosynthesis process. Anaerobic digestion (AD) biogas will not only enhance a country's energy basket status but also contribute significantly in conserving natural resources and protecting the environment (Teodorita Al Seadi DR 2008 ).

Biogas is naturally composed of biogenic material. This biogas, which occurs naturally, spreads into the ambient, and its major component, methane, plays a serious detrimental role in global warming (Bochmann and Montgomery 2013 ). Methane has been used as important fossil fuel and converted to generate power, transportation, and heating, over the past decades. Nowadays, the major portion of methane consumption and utilization comes from natural gas resources, but the production of bio-methane from waste recovery approaches has been meaningfully increased. Its production potential has been improved by 4% over 9 years (from 2010 to 2018). At present, about 3.5 Mtoe of biomethane is produced around the world and the potential for biomethane production today is over 700 Mtoe (Edenhofer et al. 2011 ). Of course, this does not mean that methane conversion is feasible from all kinds of natural resources. In other words, infrastructures for biogas development extremely rely on specific equipment and the availability of control and management systems. Therefore, a sustainable industry can be installed and implemented to generate bio-energy from renewable and green natural resources (Bochmann and Montgomery 2013 ).

Developed countries use advanced large-scale plants for utilizing biogas. Biogas is regularly applied to generate heat, power, and electricity. Also, several industrial applications for its utilization in biogas plants as a substitute to natural gas are being progressed. Based on the analyzed data, a continuous increase in biogas production has been observed due to the global policies and programs. Since 0.5% proportion of renewable energies contribution that is about 12.8 GW (IRENA RES. 2015 ) is supposed to be achieved in 2020 for transportation sectors, bio-fuel production has been considered as the main source of this plan in different regions. It is noteworthy that biogas production should not be developed as a food production threat. For this reason, biofuels are mainly generated from cellulosic and lignin wastes (Nicolae Scarlat and Fahl 2018 ; Angelidaki et al. 2018 ).

A wide global market of biogas has been conspicuously promoted for the previous decades in various countries. Moreover, the advanced biogas production technologies have been supported by domestic or international supportive rules, such as research, design, and development (RD&D) financial funds, subsidization, and guaranteed electricity purchase contracts to make a competitive market against conventional energy suppliers (Teodorita Al Seadi DR 2008 ).

According to Fig.  1 , the different utilizations of the biogas technology offer a multi-purpose solution to generate the required energy of the industrial or social sectors. Biogas is mainly consumed for combined heat and power (CHP) plants, hydrogen production units, and advanced energy systems such as fuel cells.

Overview of biogas utilization

Generally, in the European Union (EU) and North America (NA), biogas plants came to be developed more than in other continents for the last 40 years. The main advantages of the units located in the mentioned regions are industrial scale, energy efficiency, and high complexity level. Biogas production was considered by academic centers and governments owing to its potential in response to different global challenges. It should also be pointed out that using biogas technologies allows industries to eliminate greenhouse gases (GHGs) emissions and waste disposal pollutions, while it provides a broad spectrum of energy utilization such as heat, electricity, and transportation purposes, based on its renewable nature.

There are various strategies around the world for producing biogas from agricultural products. In Germany, for example, the production of cheap agricultural products that require low processing (with no outcomes for consumers) provides feedstock for biogas plants. New policies recommend the use of crops and plant residents, life stocks remaining, and landfill use (IRENA RES. 2015 ).

This review focuses on proposing a comprehensive analysis of the recent biogas technologies progress, aiming advances toward wastes conversion to produce electricity, heat, and other forms of energy carriers. It reports the current and future AD conversion technologies, as well as examines accessible details in the literature about feedstock categories, pretreatment approaches, process development, and its yield to increase production efficiency. Furthermore, suggested future biogas application trends and directions for efficient ways of energy generation from wastes are other main outputs of this study. Also, the present review highlights the emerging biogas technologies which are promoted to distribute biomethane and biofuel production, especially the production of hydrogen from biogas is the innovative insight in the mentioned field.

The structure of the present research is as follows: “Biogas Applications” reports extensive data on the up-to-date status of biogas consumption in energy generation, energy storage, and transportation. Biogas development levels around the world, regulations, and historical progress are expressed in “Biogas utilization in various parts of the world” section. Also, the characterization of the feedstocks and additives, pretreatment, process types, and related techniques are described in “Recent progress in biogas production” section. The novel technologies are indicated with their advantages and constraints for each section. Eventually, the conclusion and predictive tendencies for future research are explained in the last section.

Thus, this work represents a comprehensive review of the biogas in terms of a renewable energy source for both production and applications. The procedures for production and applications are up to date. Researchers' work in 2020 is presented where they used the most updated technologies which help other research agencies to continue from this end. The review of the development of the biogas industry and utilization covers 20 years of information. Moreover, a review of the international recent policies and regulations relevant to biogas management is provided. Based on that, a suggested policy based on international guidelines and international conventions is proposed.

Methodology

Published research papers and data on biogas sources, production, and applications are collected from the literature. These sources cover the years from 1997 till 2020 to summarize the current situation and development relevant to biogas. A review of policies and regulations on national and international levels is presented. Regulatory entities in the world that issue guidelines instruction to organize the biogas market are presented. This review showed the increase of world awareness regarding this source of energy by introducing the most updated policies in many countries. Based on all of the above, a proposed framework and policy is presented.

An introduction shows the necessity of biogas as a source of renewable energy is presented. The increasing demand for biogas in the energy section showed to be increased in the coming years. Biogas production process and the sources to get the biogas are presented. The sources vary from agricultural to animal wastes which are the richest biogas sources however, other sources such as wastewater treatment plants, and landfill disposal sites.

Applications of biogas and its contribution to the total national energy sector are presented. These applications range from energy conversion, producing alternative fuels, electricity generation, etc. Traditional methods of biogas production are presented with developments of such methods. New technologies and methods for production and purification of biogas are described.

Biogas applications

Biogas is globally considered as traditional off-grid energy. Biogas can also be utilized to generate electricity. The various applications of biogas are described below.

Electricity generation

Power generation from biomass is currently the most popular and growing market worldwide, due to technological improvements, decreasing reliance on fossil-based energy, and reduction of greenhouse gases (GHG) emissions. Biogas has the potential for electricity generation in power plants by internal combustion engines (ICEs) or gas turbines (GTs) as the two most commonly used power generation methods. Micro gas turbines are also an attractive method due to lower NOx emissions and flexibility to meet various load requirements. Multiple microturbines sizing from 70 kW to over 250 kW can be employed to meet low/medium power load demands. The electricity can provide the required power to the adjacent industries and companies. With the development of electric cars, another state-of-the-art application, especially in developed countries like Germany, is the utility of electricity for e-vehicles of a connected car-sharing association (Scarlat et al. 2018 ).

The major benefit of on-site electricity generation is to prevent transport losses and to increase reliability due to the independence from a centralized grid mostly run by traditional fossil fuels. It also brings extra economical profit by providing the required in-house power demand and selling the extra electricity (Scarlat et al. 2018 ).

Heat generation

Biogas can be directly combusted in boilers for heat generation only. It is feasible to slightly modify natural gas boilers to operate with biogas. As farm biomass is a major biogas production source, the generated heat can be used for heating the digesters, farm buildings like housing units for pigs/sties, greenhouses, as well as aquafarming, cooling/refrigeration of farm products, and drying purposes. The drying process in agricultural businesses, such as drying of digestate, woodchip, grain, herbs, and spices, is a remarkable added value to the farm economy (Herbes et al. 2018 ).

Available heat for external use, representing nearly 30–50% of generated heat, can be sold to a nearby district to be used for district heating/cooling like heating swimming pools. Also, an absorption chiller can be a potential candidate to better use heat through CHP, in addition to cooling power (tri-generation). It can convert heat into cooling power with high efficiencies of up to 70% (Rümmeli et al. 2010 ).

Combined heat and power (CHP) generation

Concurrent generation of heat and electricity by CHP systems is an operational approach to upgrade the energy conversion efficiency of biogas. When only converting biogas to electricity or heat, just a minor fraction of energy contained in biogas is used. Characteristically, in these types of systems, associated power conversion productivity is somewhere in the region of 30 to 40%, while it is diminished by employing biogas as an alternative for refined and purely natural gas (Saadabadi et al. 2019 ).

CHP plants offer the advantage of high-temperature exhaust gas from the electricity generation subsystem (ICEs or GTs) as a source of valuable heat for many heating purposes already discussed before. Although the electricity generation efficiency of simple plants is only 20–45% (Muche et al. 2016 ), a larger portion of energy (around 60% of the utilized energy (Damyanova and Beschkov 2020 )) is converted to heat that is reused by heat recovery systems; making it more attractive when there is a high heat demand. This considerably enhances the system efficiency and improves the payback period of plants, making the distributed generation the most common biogas application. The extra electricity could be supplied for the national grid and the extra heat can be sold to the local district utilization.

A CHP cycle has sufficient productivity that has an efficiency up to 90%, while it can produce 35% and 65% of the generated electricity and heat, respectively. In this case, some thermal energy is used to heat the process and about 2/3 is used for external uses. In some proposed models for biogas-based power plants, the use of generated heat is ignored and the focus is only on generating electricity. Without any doubt, this approach has no economic justification and must use all its thermal potential.

There are three common ways to produce heat and power from biogas including Gas-Otto engines, Pilot-injection gas motor, and Sterling motors (Teodorita Al Seadi DR 2008 ). In EU, four-stroke engines and ignition oil diesel engines contributed roughly the same in CHPs at somewhere in the vicinity of 50%, each (Dieter Deublein 2008 ). Biogas is also employed in gas turbines, microturbines, and fuel cells (discussed in detail in `` Fuel cells '' section ) for CHP applications (Kaparaju and Rintala 2013 ; Nikpey Somehsaraei et al. 2014 ).

CHP plants offer the advantage of high-temperature exhaust gas from the electricity generation subsystem (ICEs or GTs) as a source of valuable heat for many heating purposes already discussed. Although the electricity generation efficiency of simple plants is only 20–45% (Muche et al. 2016 ), a larger portion of energy (around 60% of the utilized energy (Damyanova and Beschkov 2020 )) is converted to heat that is reused by heat recovery systems; making it more attractive when there is a high heat demand. This considerably enhances the system efficiency and improves the payback period of plants, making the distributed generation the most common biogas application. The extra electricity could be supplied for the national grid, and the extra heat can be sold to the local district utilization. Also, an absorption chiller can be a potential candidate to better use the extra heat through CHP, in addition to cooling power (tri-generation). It can convert heat into cooling with high efficiencies of up to 70% (Rümmeli et al. 2010 ).

A CHP cycle has sufficient productivity that has an efficiency up to 90%, while it can produce 35% and 65% of the generated electricity and heat, respectively (Shipley et al. 2009 ). In some proposed models for biogas-based power plants, the use of generated heat is ignored and the focus is only on generating electricity. Without any doubt, this approach has no economic justification and must use all its thermal potential.

Upgrading to biomethane

If biogas is upgraded and purified to biomethane, it can be fed into natural gas grid to be used for heating purposes, power generation, or to provide fuel for compressed natural gas (CNG) and even natural gas vehicles (NGV). A significant benefit of biomethane is that it can be stored to meet peak demands (Herbes et al. 2018 ). The two major steps to produce biomethane are upgrading methane content up to 95–97% followed by a cleaning process to eliminate water vapor, hydrogen sulfide, oxygen, ammonia, siloxanes, carbon dioxide, carbon monoxide, hydrocarbons, and nitrogen (Ryckebosch et al. 2011 ). Biogas upgrading is performed by physical and chemical technologies such as adsorption, absorption, cryogenic and membrane separations, and gas separation membranes as well as biological technologies (in situ and ex situ (Kapoor et al. 2019 )). Although biological methods are emerging, suggesting an enormous technological potential, they are not widely used in industry since they are generally much slower, have low rates of reaction/synthesis, and require long startup period that made them less economically feasible, while physicochemical methods are common due to technological advancements and implementations (Scarlat et al. 2018 ).

Upgrading biogas to biomethane or renewable natural gas (RNG) is on a hot trend in developed countries especially in North America among oil and gas companies for decreasing GHG emissions and using the carbon credit. There are also other environmental and economical benefits in smaller scale to farmers, municipalities, and counties for waste management and profitable contracts with gas utility companies. Biomethane market for transportation purposes equaled to 160 cubic meter per year in 2015 Eurostat.European Statistics ( 2019 ).

Transportation fuel

Biogas converted to biomethane (through upgrading and cleaning) can be readily used in natural gas-powered vehicles as another option for fossil natural gas. Using biomethane as transportation fuel results in remarkably low GHG emissions that make it a suitable source of renewable fuel. Biomethane turns out to be a great fit to replace fossil-based fuels in terms of environmental and economic considerations (Scarlat et al. 2018 ). However, the overall efficiency is extremely improved when biomethane is utilized in advanced hybrid or fuel cell vehicles (FCVs) in comparison to current biodiesel or ethanol-powered ICE vehicles (Faaij 2006 ).

Generally, biogas can be improved to transportation fuels (bio-CNG) that can be stored for future use, in the form of liquefied biogas (LBG), syngas/hydrogen, methanol for gasoline production, ethanol, and higher alcohols (Yang et al. 2014 ). Compression and liquefaction are common physical methods to convert biogas into bio-CNG and LBG, while the dominant chemical approach to obtain syngas is catalytic reforming. If Fischer–Tropsch synthesis (FTS) or fermentation is employed, syngas may be converted into a variety of alcohols like methanol, ethanol, and butanol (Yang et al. 2014 ). This fuel alternative has already been applied within the European Union and the USA. As an example, many vehicles run on biogas in the urban public transport (in Sweden and Germany) either as 100% methane (CBG100) or mixed with natural gas (e.g., CBG10 and CBG50) (Damyanova and Beschkov 2020 ; Yang et al. 2014 ).

Hydrogen production

Hydrogen displays many promising potentials for renewable energy and the chemical industry due to its high potential for energy production. Hydrogen offers the biggest share of energy per unit mass (121.000 kJ/kg). The hydrogen council suggests about 18% contribution of total final energy utilization by 2050. Hydrogen is best employed in fuel cells as an emerging energy application to produce electricity, heat, and possibly water. Furthermore, there are many applications in chemical industries for hydrogen, including food treatment, hydrogenation methods, production of ammonia and methanol, Fischer–Tropsch synthesis, pharmaceutical manufacturing, among others (Armor 1999 ).

Technically, hydrogen (H 2 ) can be released from the BSR (biogas steam reforming) process. This process has temperature flexibility in the range of 600 to 1000° C, which also includes catalytic techniques. (Holladay and J., King, D.L., Wang, Y. 2009 ; Alves and C.B., Niklevicz, R.R., Frigo, E.P., Frigo, M.S., Coimbra-Araújo, C.H. 2013 ). The main difference between BSR and SMR (steam methane reforming) is the presence of carbon dioxide in the feedstock. This factor increases the sensitivity to carbon production in the process. The produced carbon can deposit in the active phase of the catalyst to create deactivation.(Gioele Di Marcoberardino et al. 2018 ). Furthermore, fed gas can affect the hydrogen separation unit. In this case, PSA (pressure swing absorption) and VPSA (vacuum PSA) are the most common methods of purifying the system for hydrogen-rich reformate or syngas (Ugarte and P., Lasobras, J., Soler, J., Menéndez, M., Herguido, J. 2017 ; Ahn and Y.W., Lee, D.G., Kim, K.H., Oh, M., Lee, C.H. 2012 ). The potential of hydrogen production from all landfill sources in the USA is probably between the total potential of 16 million tons of methane from raw biogas and 4.2 million tons of hydrogen (Milbrandt GSaA. 2010 ). Biogas production systems have a capability for production from 100 Nm 3 /h for small-scaled agricultural to a few 1000 Nm 3 /h for large-scaled municipal waste landfills; furthermore, occasionally, not all biogas may be converted to the desired hydrogen and further biogas valorization can coexist in the system. Therefore, the capacity considered for BSR should be in the range of 50 and 1000 Nm 3 H 2 /h (Doan Pham Minh et al. 2018 ).

Hydrogen is clean transportation fuel, while as discussed earlier syngas may be used as a feedstock for alcohol production. With new advancements in reforming procedures, biogas can now be directly improved to syngas by dry or steam reforming without the necessity to remove carbon dioxide (Yang et al. 2014 ).

Fuel cells are probably the cutting-edge application of biogas. Recent advances in fuel cells resulting in low emissions (CO 2 , NO x ) and high efficiency make them suitable for power generation and transportation purposes. Also, fuel cells can be utilized in large-scale power plants, power distribution generators, buildings, small-scaled and portable power supply apparatus for microelectronic equipment, and secondary power components in vehicles (Alves et al. 2013 ).

Fuel cells can use the chemical energy of hydrogen and oxygen without any intermediaries to deliver electricity and heat (A. Trendewicz R.B. 2013 ). In this case, there are only a small number of fuel cell-based power plants (most of which are pilots) that generate electrical power from biogas. (S. Ali Saadabadi ATT, Liyuan Fan, Ralph E.F. Lindeboom, Henri Spanjers, P.V. Aravind. 2019 ). Fuel cells exhibit high electrical efficiency of 60% (in power generation only mode) and thermal efficiency of up to 40% (in CHP applications) (Pöschl et al. 2010 ), but can easily be integrated with other power generation systems like gas turbines or microgas turbines to further improve their performance. Also, biogas fueled integrated solid oxide fuel cell (SOFC)-CHP offers a modern energy system that can address both heat and power generation demands for decentralized grids with drastically higher electrical efficiencies (Wongchanapai et al. 2013 ; Safari et al. 2020 ; Safari et al. 2020 ). Such high efficiency compared to other common combustion technologies is a result of not being limited by thermodynamic Carnot efficiency. SOFCs are more tolerant to fuel impurity and flexibility; hence offering better integration with biogas systems (Wasajja et al. 2020 ). This highlights their key role in enhancing the highly efficient generation of electricity from biogas, which demonstrates significant environmental and economic merits. However, for the use of biogas as fuel in fuel cells, a cleaning procedure seems essential to eliminate biogas impurities such as H 2 S, siloxanes, and other volatile organic compounds (VOCs) that have harmful impacts on fuel cell operation.

Furthermore, hydrogen produced from biogas can directly feed fuel cells. The reforming practice can be succeeded either internally employing fuel cells or externally by a catalytic pre-reformer. The three chief techniques for methane conversion are steam reforming, partial oxidation (POX), and dry reforming. Besides, mixed approaches like autothermal reforming (ATR) (mixed steam reforming and methane POX) are applicable. In a pilot plant constructed in Barcelona, Spain named “Biocell project”, biogas from a WWTP was employed in two categories of a fuel cell. The first was proton-exchange membrane fuel cell (PEMFC) that entailed exterior gas cleaning and reforming unit. Biogas has also been added into a SOFC after the cleaning process. This pilot plant is intended for 2.8 kWe. Electrical and thermal effectiveness for the SOFC pilot plant was 24.2 and 39.4%, respectively, which are considerably more than those for the PEMFC pilot plant (S. Ali Saadabadi ATT, Liyuan Fan, Ralph E.F. Lindeboom, Henri Spanjers, P.V. Aravind. 2019 ; Arespacochaga and CV, C. Peregrina, C. Mesa, L. Bouchy, J. Cortina 2015 ).

Biogas development in various parts of the world

The worldwide biogas industry has increased more than 90% between 2010 and 2018, while further growth is still expected. The International Renewable Energy Agency (IRENA) reported that the overall potential for the biogas industry in 2018 could provide 88 Tera Watt per hour (TWh) of biogas each year. Installed electricity generated from biogas reached 18.1 GW in 2018, against 8.2 GW in 2009 (Agency 2019 ). Over 20% of electricity produced in the entire biopowered production is generated from biogas, with a share of 4% of heat generation worldwide.

Among different countries throughout the world, Europe plays a pivotal role in biogas electricity generation. In 2017, Europe contributed to over 70% of the world biogas generation representing 64 TWh, followed by North America accounting for 15 TWh (in which the US participation was over 85% in entire North America). Asia produced 4 TWh followed by Eurasia with 1.7 TWh, South America with 953 GWh, and Africa biogas production accounted for 89 GWh (Scarlat et al. 2018 ; Agency 2019 ).

In terms of thermal energy production, biogas is turning to be a more significant source of heat, in which around 4% of the worldwide bioheat in 2015 was generated by biogas. In the EU, biogas produced 127 TJ of heat, which corresponds to almost 50% of entire biogas use in the EU (Scarlat et al. 2018 ). In Demark, the electrical power cost produced by biogas is 0.056 EUR/kWh in a CHP unit or injected into the grid (Seadi and J. 2019 ).

Biogas utilization differs significantly in various countries around the world. This varies from several small-scaled biogas plants providing heat in China and India to large-scale plants generating electricity as well as upgrading into biomethane as fuel, mostly in Sweden (McCabe et al. 2018 ).

Nanyang in China is one of the top biogas cities in the globe due to its location in the center of a rank soil zone. Since corn is abundant, other types of cereals can be employed for producing biogas (Dieter Deublein 2008 ; Lei Zheng 2020 ).

In China, biogas plants are classified as medium scale with the volume of digester equaled to 300 cubic meters and large scale with a capacity of 500 cubic meters, with daily biogas production in the range of 150 to 500 cubic meters per day (Song and C., Yang, G., Feng, Y., Ren, G., Han, X. 2014 ). The governmental support for domestic digester has been stopped since 2015. More backing would make large-scale biogas plants and bionatural gas schemes (Ndrc 2015 ). Chinese biogas industry reported that 41.93 million biogas digesters were built (containing centralized biogas source for houses), for almost 200 million recipients, in which 14.5 billion m 3 biogas is produced per year (China Statistics Press 2018 ).

In India, around 2.5 Mio biogas plants are operating, with a medium digester volume of 3–10 m 3 . Based on the circumstances, the plants produce 3–10 m 3 biogas daily, adequate to deliver a regular farmer family with energy for food preparation, heating, and lighting. Also, more than 1.2 million households employ small-scaled AD and 100,000 family-sized AD units have been installed between 2016 and 2017. Over 35,000 biogas plants have been constructed with governmental investments (MNER 2016 ).

Japan is a pioneer in the use of biogas, with increasingly using AD to produce biogas and manage municipal waste in the last decade. The development is such that only Japan uses thermophilic AD (Abbasi et al. 2012 ).

Up to 2008, over 70 plants have been constructed in Russia, over 30 in Kazakhstan, and a single plant in Ukraine. In Ukraine, bioreactors with 162,000 m 3 volume have been previously installed in sewage treatment units (M. R. Atelge DK, Gopalakrishnan Kumar, Cigdem Eskicioglu, Dinh Duc Nguyen, Soon Woong Chang, A. E. Atabani, Alaa H. Al-Muhtaseb, S. Unalan. 2018 ).

It should be noted that some nations employed biogas as a practical tool for waste management, mostly to decrease the detrimental effects of municipal waste or wastewater. Likewise, a broad range of various technologies are employed from simple digesters to expanded granular sludge blanket (EGSB) digesters (McCabe et al. 2018 ).

Biogas technology and industry

The biogas industry varies significantly in the various parts of the world. Different countries have been advanced in several types of biogas systems mainly premised on different environment as well as energy demand and supply chain. The UK, Australia, and South Korea employed landfill sites to achieve a considerable portion of their produced biogas, while in Switzerland and Sweden, using decomposition of sewage to generate biogas is prevailing. Denmark utilizes mainly manure due to its abundance and availability. In Germany, UK and Sweden most of the biogas generation arises from food waste (McCabe et al. 2018 ; Union 2015 ; Association WB.Global Potential of Biogas 2019 ).

In farm-based biogas production, China and Germany are recognized as world leaders since about 24,000 small-scale plants exist in China and nearly 8000 agriculture plants in Germany. Similarly, France, Holland, Austria, and Italy employed considerable farm-based biogas plants (Union 2015 ). Moreover, the scale of plants ranges from small household units to larger plants using feedstocks such as household waste, industrial waste, and manure to generate both heat and electricity (Union 2015 ). Studies revealed that in Asia and Africa, most of the installed biogas plants were family-sized (Kemausuor et al. 2018 ). China and India have dominated the microscale biogas industry in the world. At this time, Thailand takes benefits from more than 1700 biogas plants and more than 150 plants of industrial waste. The Thai government has attempted to expand industrial wastewater technology that has the potential of 7800 TJ/y biogas production (Tonrangklang et al. 2017 ). The ministry of energy of Nepal (Government of Nepal Ministry of Energy WRaI.Biogas. 2020 2020 ) has reported that most of the villages about 2800, out of the total 3915 in all 75 districts of Nepal, have small-scale or household biogas production systems. Primarily two categories of plants have been constructed in Nepal. These are the floating-drum plant based on the Indian style and fixed-dome plants with a flat floor, cylindrical digester, and a dome prepared by concrete. Among 50 million microscale digesters operating in various parts of the world, 42 million are installed in China and another 4.9 million in India. The statistics from the World Biogas Association (WBA) have shown that there are only 700,000 biogas plants installed in Asia, Africa, and South America (Association WB.Global Potential of Biogas.2019. 2019 ).

In terms of large-scale plants, about 7000 large-scale biogas systems are operating in China. Europe, in 2017, had a share of 17,783 plants, while Germany was dominating the European biogas industry with 10,971 plants followed by Italy with 1665 plants, France with 742, Switzerland, and the UK with 632 and 613 plants, respectively (Association 2018 ). The World Biogas Association data mentioned about 2200 anaerobic digesters large-scale plants in the USA, able to generate 977 MW (Association WB. International Market Report 2018 ).

Another application of biogas relies on upgrading to biomethane. Although being comparatively a novel technique, it achieves widespread utilization worldwide. Some biogas upgrade plants are employed to produce vehicle fuel, while others deliver it into the local or national grids Association WB.Global Potential of Biogas ( 2019 ).

Africa is a region with abundant and diverse resources for biogas production, though it has accomplished small progress in the sector. Although the continent has made considerable achievements in small-scale biogas plants, profitable biodigesters still require further development (Kemausuor et al. 2018 ). In Africa, harvest and livestock farmers, small to medium and large food treating businesses, wastewater, sanitation, and municipalities running institutes, as well as municipal waste management organizations, are considered as potential candidate employers of large-scale biogas technology. Moreover, schools, institutions of higher education, hospitals, and commercial buildings have the potential to benefit from biogas technologies and facilities (Parawira 2009 ). Excluding South Africa, insufficient scientific literature has reported technology development of the commercial biogas system in Africa. In the Southern parts of Africa, developed technologies are the lagoon, plug low, and up-flow sludge blanket (UASB) (Mutungwazi et al. 2018 ).

Biogas production and utilization

In this section, biogas production from wastewater treatment plants (WWTP), biowaste digestion, agricultural products (largely manure and energy crops), waste stream from different industries, and landfill gas are considered. In Europe, Germany has dominated the industry by far in which its annual production is accounted for 120 TWh followed by the UK with 25 TWh and 9 TWh in France. Denmark and the Netherland's production capacity is around 4 TWh and the remaining countries share is less than 3 TWh (Bioenergy 2019a ).

In Germany, the total gross electricity and heat production from biogas is about 33 TWh/year and 18.8 TWh/year, respectively. Based on statistics revealed by the Federal Ministry for Economic Affairs and Energy of Germany, a considerable amount of the biogas was utilized for electricity production (58%) and heat production (33%), and approximately only 1% was used as a vehicle fuel (Bioenergy 2019a ).

In 2018, about 32% of entire renewable heat used in the UK was produced by anaerobic digestion technology, of which 9 TWh/year was produced by biomethane, 2 TWh/year by biogas and CHP accounted for 918 GWh/year, while 2681 GWh of electricity was generated by the sector (Association ADaB. ADBA annual report 2019. 2018 ).

In France, total electricity production from biogas was about 1.8 TWh/year at the end of 2017, simultaneously total heat generated accounted for 1.7 TWh/year, which demonstrates nearly equal portion for both heat and electricity. Regarding heat production, the agriculture sector accounts for an indispensable portion, while in electricity production, the landfill has a pivotal role with 953 GWh/year followed by agriculture with 765 GWh/year (Bioenergy 2019a ).

In Denmark, the biogas sector provides 5% of the entire energy consumption of which biogas plants contribution is 60% and the rest relies on wastewater treatment plants and landfill sites. The Danish Energy Agency states that due to several support schemes such as upgrading biogas to Natural gas, biogas employment for process purposes in the industrial sectors, etc. results in promoting biogas utilization through the country (Agency and Biogas in Denmark 2019 ). Total Danish biogas production at the end of 2018 was reported to be about 1763 GWh/year in which the agriculture sector (both centralized and farm plant types) showed the largest contribution with 1367 GWh/year. 66% of produced biogas energy (which corresponds to 1150 GWh) is used to provide electricity, followed by upgrading plants with 17% portion and heat generation with 16% Bioenergy IEAI.Denmark Country Report -2019 ( 2019 ). In the Netherlands, in 2017, two co-digestion and municipal waste plants had the largest share in production, and the final use of biogas (3034 TJ heat was produced solely with municipal waste, while co-digestion had a pivotal role in electricity production representing 1825 TJ) Bioenergy IEAI.The Netherlands Country Report -2019 ( 2019 ).

In Sweden, 48% of biogas production corresponds to co-digestion plants followed by WWTPs (37%), the remaining being produced by the other plant types such as landfills, industrial facilities, and farm-based. In terms of utilization, the upgrading or transport sector represented a considerable portion (65%) followed by heat (19%), while electricity production share was almost 3% (Bioenergy 2019b ).

In Asia, China plays a significant role with 98.4% of biogas production between non-OECD countries. Primary infrastructures such as advanced industry and socioeconomic conditions have a profound impact on biogas generation and utilization growth. Small-scale and household biogas systems have been widely developed by countries like India and Bangladesh. Various researches prove that there are plenty of resources for producing biogas in developing countries when barriers such as socioeconomic, climate conditions, and appropriate technology have been addressed accurately. Several biogas plants in the range of medium to large scale have been launched in China and India (Mittal et al. 2019 ; Jiang et al. 2011 ; Gu et al. 2016 ).

In the USA, over 2200 biogas plants are operated, among which 250 AD on farms, 1269 wastewater recovery plants employing an AD, and 66 independent plants that use food waste as feed and 652 landfill gas projects. The America Biogas Council has revealed that there is still an enormous potential for developing the biogas industry in the USA where it is possible to achieve 103 trillion kWh/year (Council 2019a ). California ranks first in biogas production potential among all the 50 states in the USA (Council 2019b ), followed by Texas (Council 2019c ).

The power generation from biogas is estimated to be 9731 million kWh and 6574 million kWh electricity for California and Texas states, respectively. In California, the manure system has the highest potential with about 900 biogas plants, while currently 38 manure plants are operated with 156 wastewater facilities in Texas. (Council 2019b , c ).

In Canada, bioenergy currently provides approximately 26.7% of Canadian entire renewable energy market, the highest share is from burning solid biomass (23.1%), followed by the liquid biofuels (2.4%), and biogas (1.2%) (Canada 2019 ). In Canada, total installed plants for biogas production are estimated to be around 150. Most production takes place in landfills with 45 plants (share of 30%), followed by the agriculture sector with 37 plants (share of 24.7%) and WWTPs with 31 plants (20.7% production portion) (Association WB.Canada Market Report. 2019 . 2019 ).

Based on the Canadian Biogas Association data, at the end of 2018, about 195 MW of electricity and 400,000 GJ of Renewable Natural Gas (RNG) were generated (Biogas and Potential. 2019 . 2019 ). Biogas is utilized for providing heat and electricity, delivering to a nearby user using a pipeline, converting into electricity and connecting to the grid, or refining to RNG based on circumstances such as the landfill site location, and the energy demand of plants. In this regard, approximately 50% of the produced biogas is converted into power, with the rest going to combined heat and power (CHP) application (about 25%), heat (only 10%) and RNG (about 4%), and electricity and RNG (about 1%) (Association WB.Canada Market Report.2019. 2019 ).

In Australia, at the end of 2017, generated electricity from biogas industry was approximately 1200 GWh, which is equivalent to almost 0.5% of the entire electricity generation of the country, while biogas potential electricity generation was estimated as 103 TWh, equal to almost 9% of Australia’s entire energy consumption (Australia and Biogas opportunities for Australia. 2019 ).

The main use of biogas in Australia is for electricity with the greatest share for landfills (53.7%), followed by biowaste and WWTPs (40% and 33.3%, respectively). Heat is used in the industrial sector with a share of 30% and afterward the WWTPs with a share of 26.2%. In CHP applications, agriculture plants have the largest portion (50%), followed by the biowaste and the WWTPs (equal share of about 20% each). Between 40–50% of the excess biogas is flared at agriculture, industries, and landfills. Twenty percent of WWTPs and biowaste are no biogas upgrading plants in Australian’s biogas industry (Bioenergy IEAI.Australia Country Report. 2019 . 2019 ).

In Africa, South Africa has the largest share of installed biogas plants with about 700 plants, while only 300 plants might have been in operation as of 2007, while it can generate 148 GWh electricity from estimated biogas potential by appropriate investment and implementation schemes (Kemausuor et al. 2018 ).

Various industrial trends in the biogas production have been introduced to improve quantitative and qualitative properties of the biogas. Yet, the accomplishments of AD intended for advanced investments will increase from the low charge of feedstock accessibility and the broad range of practical set ups of the biogas (i.e., heating, electricity power, and fuel form). The remained parts of slurry from biogas production procedure have the potentials to be improved to be used as fertilizer to enhance the sustainability. Produced biogas could be employed to generate power for integrated or isolated systems in the rural and urban regions and are deemed to be economical favorable. The employed processes of AD, modern trends accompanied by included advantages and disadvantages are also demonstrated more details and progress on the way to producing biogas in a sustainable approach. Obtained results from previous researches indicated that the present amount of biogas production confirms that regarded approaches would have main influence on the energy utilization in upcoming times. The impression contains diminished release of pollutants to the atmosphere guarantees that the global warming prevention. Nevertheless, the current trend of the biogas production varies in diverse countries, either in production or the sources (landfill, AD, sewage sludge, or thermochemical methods). The involvement of biogas to the domestic natural gas utilization varies differently, around 4% on standard values; however, it raised 12% in Germany. The major nations in the biogas production in the European Union are France, Italy, Germany, Czech, and UK. Germany stands as the European frontrunner with a biogas production of 329 PJ and a contribution of 50% of total in the EU. It’s reasonable to surmise that, based on the provided data from various researches, it has been declared that given the growing need and available technology, European Union countries, and especially Germany and Sweden, will be pioneers in the development, operation, and production of biogas in the world. Table 1 indicates the biogas plants, upgrading units, and their upgrading capacities in certain EU countries (Lampinen 2015 ; Backman and Rogulska 2016 ; Esmaeilion et al. 2021 ).

Recent progress in biogas production

Producing biogas is a key option in the energy sector of various countries. There is a wide variety of raw materials for utilization in biogas plants. In this case, obtaining a stable state in plants is a crucial concern that influences the prices and additives. Another important issue in the biogas plants is that their products should be attractive in terms of value and efficiency (Chen et al. 2012 ). Recent progress in the field of biogas production can be divided into three categories: feedstock and additives, pretreatments, and processes.

Feedstock and additives

The organic matters are the main feedstocks in the biogas plant, which can fall into different categories. Evaluating the potential of biogas production based on organic matters from rural regions has been investigated. The highly fermentative wastes can decrease the quantity of feedstock in biogas plants (Pawlita-Posmyk and Wzorek 2018 ).

Microalgae with satisfactory features is a potential option for feedstock in biogas systems. In comparison with other biomass resources, microalgae has better efficiency, more convenient production, and higher content of lipid and polysaccharide that make it a flexible choice in biogas plants (Wu et al. 2019 ). Kaparaju et al. (Kaparaju et al. 2009 ) explored the production of biogas from sugars released from wheat straw with the aid of hydrothermal pretreatment based on the biorefinery procedure. In this case, the pretreatment process increased the gas yield by 10%.

For achieving sustainable progress, the global trend of energy production is moving to the waste-to-energy (WTE) method which has multilateral benefits. Currently, biomass resources are being employed to generate energy. All around the world, biomass satisfies around 50 exajoule of the entire energy demand annually (Steubing et al. 2010 ; Ferreira et al. 2017 ; Ahmadi et al. 2020 ).

A broad spectrum of waste types can be consumed as a feedstock in biogas units by anaerobic digestion (AD) technology. Huge amounts of lignocellulosic waste could be collected from agricultural and municipal resources. The most common types of waste and residuals that can be used in the biogas sector are animal manures and dungs, muck and slurry, domestic/municipal wastewater (sewage), mud (sludge), urban garbage or municipal solid waste (MSW), and food substances loss. Table 2 indicates the power generation and associated yields of biogas production by accessible resources (Waste-to-energy 2015 ; Stucki et al. 2011 ).

To enhance the yield of biogas production, utilization of additives is an acceptable method. Specifications of these components can be varied based on their biological or chemical properties under various conditions. With the aid of these materials, desirable conditions for bacteria could be provided. However, biocenosis features are vital for achieving the ideal concentration (Demirel and Scherer 2011 ).

Using salts with Mg and Ca improves methane production efficiency with low slurry foaming (Sreekrishnan et al. 2004 ). For stabilizing pH fluctuations and reducing the contents of NH 3 and H 2 S, several types of additives have been studied (Kuttner et al. 2015 ). Furthermore, using zeolite compounds has the potential to intensify the quantity of biogas production by 15%, also the addition of CaCO 3 can improve this yield by 8%. Adding biological additives increased the production rate of biomethane and biogas by optimizing AD (Vervaeren et al. 2010 ). Using biological additives is a common way of increasing biogas production yield. Yi Zheng et al. (Zheng et al. 2014 ) stated that by adding enzymes to lignocellulosic biomass, biogas production was enhanced by 34%. Vervaeren et al. (Vervaeren et al. 2010 ) reported that by adding homo and hetero-fermentative bacteria to maize components, production yield increased by 22.5%. With the addition of fungi compounds (e.g., ceriporiopsis subvermispora ATCC 96,608) to the yard trimmings, methane production increased by 154% (Zhao 2013 ). The alternative options for biological additives are chemical compounds. Using a wide variety of chemical additives like NaOH, Ca(OH) 2 , NH 4 OH, H 3 PO 4, etc., can improve the associated biogas production yield. Chandra et al. reported the effects of using NaOH as an additive to the wheat straw. Obtained results presented that yield of methane could be improved by up to 112% (Chandra et al. 2012 ). Badshah et al. investigated the diluted H 2 SO 4 properties, added to the sugarcane bagasse, which could increase the production rate by up to 166% in comparison with pre-additive treatments (Badshah et al. 2012 ).

The impact of activator addition on the biogas quality slurry is investigated in Indonesia (Ginting 2020 ), the study started by adding new bioactivator prepared from agricultural wastes such as bananas, papayas, and pineapples waste with an additional of chicken intestines where the bacteria in the chicken intestine are effective at work. The addition of the activator resulted optimally in the work where stable gas production was achieved. The slurry at the end of the production process was a liquid fertilizer ready to use. The study showed the best concentration of the activator in the production process of both the slurry and the biogas.

Pretreatment

Predominantly, there are two wide-ranging classifications for biogas production upgradation, ex situ, and in situ techniques, while most of the methods focus on ex situ approaches. Some of the conventional ex situ treatments are adsorption, catalytic processes (e.g., biological or chemical), membrane gas permeation, desulfurization, scrubbing, and absorption. Sarker et al. ( 2018 ) overviewed the in situ biogas production upgrades.

With the help of the in situ method, the associated cost concerning cleaning techniques could be reduced and the quality of produced biogas improved in the same vein. Nevertheless, the in situ method is limited to the empirical state and prototype models. Figure  2 summarizes various types of biogas upgrading methods (Sarker et al. 2018 ; Bassani et al. 2016 ; Rachbauer et al. 2016 ; Lemmer et al. 2015 ).

Biogas improvement by ex situ and in situ techniques (Sarker et al. 2018 ; Bassani et al. 2016 ; Rachbauer et al. 2016 ; Lemmer et al. 2015 )

The pretreatment productivity influences the associated bioprocess efficiency of lignocellulose. Pretreatment techniques are intended to make AD faster, enhancing the yield of the biogas, and producing a broad range of usable substrates.

Figure  3 indicates the mentioned effects of pretreatment processes. By considering efficiency, economy, and application as objective functions, optimization of pretreatment processes is a necessitated aim. Pretreatment should be operative in eradicating the structural obstacles of associated polymers with lignocellulose (it should be noted that the cellulose and hemicellulose constituents are in this classification), through exposing these substances to microbial decay efforts, which increases the biomass degradation and consequently enhances the biogas yield (Spyridon et al. 2016 ).

Pretreatment effects on the value of anaerobic digestion ( b ) and yield of CH 4 ( c ) (Achinas et al. 2017 )

There are crucial requirements in common designs of biogas plants for increasing the rate of gas production. Recently, innovative designs of biogas plants have been introduced (e.g., Konark, Deenbandhu, and Utkal Models) (Sreekrishnan et al. 2004 ; Kalia and Singh 2004 ; Abouelenien et al. 2010 ; Prasad et al. 2017 ) in which the design parameters changed to increase productivity and effectiveness in cost factors. In these concepts, by implementing optimum measurements in regarded shapes (similar to the spiral shape), the index of gas storage volume was enhanced by 33–50%, while the related costs were reduced by 10–15%.

The hydrolysis of a high proportion of non-biodegradable compositions from MSW (which is intractable by AD) can be performed by microwaving or autoclaving (Pecorini et al. 2016 ). In another study, by applying pressure to biowaste in the pretreatment procedure, biogas yields were improved significantly (Micolucci et al. 2016 ). The most desirable condition in the pretreatment of biomass is to provide an ideal environment for breaking down the feedstock substances to the sugars that are fermentable, by increasing the accessibility for microorganisms. This process leads to eradicating the lignin endurance and declining the cellulose’s crystalline formation (Micolucci et al. 2016 ). Table 3 presents the merits and demerits of various pretreatment technologies.

By implementing fast pyrolysis pretreatment, biogas production has been increased (Wang et al. 2016a ). This innovative approach in thermochemical pretreatment with the aid of a lower temperature fast pyrolysis (LTFP) to enhance the performance of the AD process has been introduced, in which corn stover was used as a primary substance.

During the pretreatment procedure, a fluidized bed pyrolysis reactor applied high-temperature gas flow at 200 °C. To improve the efficiency, different strategies in the pretreatment section were performed (e.g., characteristics analysis, assessing crystal concentration of the corn stover components). Comparing the results obtained between pre- and post-treatment, the production efficiency of methane increased by about 18%. In thermochemical pretreatment, chemical bonds in substances would be broken by implementing the thermo-physical process. Biogas production and hydrolysis of celluloses are affected by the degradation of hemicellulose and lignin (Cara et al. 2006 ). Thus steam explosion falls into this category (Bauer et al. 2014 ). In this method, biomass is subjected to high-temperature steam at 240 °C, so that after a long time, morphological and chemical transformations in biomass can occur (Biswas et al. 2011 ). Another pretreatment method to upgrade the biomass is the Torrefaction process which is applied to produce a higher amount of hydrophobic fuel with a fixed range of carbon content. The operational temperature for this process is from 200 to 300 °C in a stable environment (Mafu et al. 2016 ; Sarkar et al. 2014 ). Fast pyrolysis is an additional pretreatment that was highly used in the field of biofuel production. In this case, by reducing the temperature (around 200 °C) lignin and hemicellulose could be wrecked. Nonetheless, there is no study demonstrating an increase in biogas production (Bridgwater 2012 ; Y-m et al. 2009 ). Rodriguez et al. (Rodriguez et al. 2017 ) investigated different pretreatments for grass in biogas production sectors. The obtained results revealed that all pretreatments could increase biogas production by around 50% even though all of them suffer from high energy consumption.

The ultrasonic pretreatment process is an innovative and practical technique in the pretreatment section. This process increases the efficiency of sludge dewatering, stability of the digestion, solids solubility, and rate of biogas production. The outcome of this method is a digestate containing a low share of residual organic materials. The ultrasonication modifies the biological, chemical, and physical specifications of the sludge. Some of these variations are pathogen reduction, settling velocity improvement, and protein concentrations increase (Cella et al. 2016 ; Liu et al. 2015 ; Feng et al. 2009 ).

By applying this pretreatment, the rate of CH 4 production increased by 34% (up to 80% of energy consumption in the pretreatment unit is reachable by produced methane) (Mirmasoumi et al. 2018 ). The Lysis centrifuge consists of a method focused on centrifuge which initiates partial destruction in sludge cells. This strategy can improve biogas production by 15–26% with thickened sludge resources. This practice is suitable in pretreatment processes (for dewatering) and does not impose any extra load on the system for extra operations (Dohányos et al. 1997 ).

Biological pretreatment is an alternative for thermal and chemical pretreatment that is composed of different stages like enzymatic hydrolysis, using fungi additives and thermal phased AD (TPAD). Among named processes, TPAD has attracted attention. The benefits of this biological pretreatment are lower energy consumption and higher biogas production in comparison with other methods (Zhen et al. 2017 ; Bolzonella et al. 2012 ).

By comparing the results between thermal and autohydrolysis pretreatments, the production of biogas in the biological procedure is considerably lower than in the thermal pretreatment (26% and 45%, correspondingly). The dominant conditions of autohydrolysis pretreatment were reported to be at 55 °C for 12–24 h compared with 170 °C for half an hour for thermal pretreatment (Carvajal et al. 2013 ). In this field, the highest yield achieved in biogas production was investigated by Bolzonella et al. (Bolzonella et al. 2012 ) by applying the pretreatment at 70 °C for 2 days, with associated yield increasing by up to 145%. It is noteworthy to mention that many studies have investigated the combined pretreatment for increasing the biogas production yield (Liu et al. 2018 ; Bao et al. 2015 ; Chan et al. 2016 ; Abelleira-Pereira et al. 2015 ; Wang et al. 2014 ; Bentayeb et al. 2013 ) however, this is out of the scope of this study.

The biogas production can be categorized into two main fermentation process which are dry and wet processes. For the digestion by wet process, the overall solids concentration in the fermenter is lower than 10%. To treat solid substrates, using liquid manure for achieving pumpable slurry is necessary. On the other hand, in the dry digestion, the overall concentration of solids in the fermenter is ranging from 15 to 35%. The stability in the wet digestion processes is higher than in dry methods. In the agricultural section, wet digestion practices are more widespread (Weiland 2010 ).

The biogas production procedure includes four important phases which are hydrolysis, acidogenesis, acetogenesis, and methanogenesis as can be seen in Fig.  4 .

Diagram of the biogas production procedures by AD (Mao et al. 2015 ; Visvanathan and Abeynayaka 2012 )

For developing methane fermentation, diverse associations of bacteria are needed, which are aceticlastic and hydrogenotrophic methanogens, syntrophic acetogens, fermentative bacteria, and homoacetogens. The balanced contribution between them increases the efficiency of biogas production and the AD process (Chen et al. 2016 ). There is a specific type of AD that involves anaerobic membrane bioreactors (AnMBRs), which increases the quantity of biogas production by membrane specifications. By considering the techno-economical parameters of AnMBRs, the efficiency of biogas production has the potential to be increased dramatically (Chen et al. 2016 ). Figure  5 shows the different types of AnMBR technologies.

The biogas production processes by different types of AnMBR technologies

The methanogenic organisms have a negative instinct for sluggish growing, and also the complexities of microbial in the systems have caused difficulty in the functioning of biogas fermenters. An innovative concept of integrating the anaerobic bioprocess with membrane breakdown practice through a membrane bioreactor (MBR) allowed augmenting the biomass concentration through a bioreactor. With an anaerobic membrane bioreactor (AnMBR), high hydraulic load, and adequate mixing brought sustainability for high cell concentrations (Wang et al. 2011 ).

The AnMBRs have a special feature for providing satisfactory retention of active microorganisms. This specification leads to optimal productivity and favorable resistance against toxic substances. Furthermore, high concentrations in the final product and easy separation of biomass and products (by micro-/ultra-filtration) have been added to its benefits (Ylitervo et al. 2013 ). Obtained results revealed that methane yield in biogas production was up to 0.36 l CH 4 /g chemical oxygen demand (COD) and methane content reached 90% (Liao et al. 2010 ).

Wang et al. (Wang et al. 2011 ) discussed the developing approaches for the biogas sector in China and presented every aspect of this technology including the AnMBRs. Ylitervo et al. reviewed the MBR strategy for producing ethanol and biogas and explained the progress in MBRs (Ylitervo et al. 2013 ). Minardi et al. (Minardi et al. 2015 ) reported various applications of the membrane in biogas technologies and purification methods. Mao et al. (He et al. 2012 ) investigated the latest trends in biogas production by AD and AnMBRs. To improve the efficiency of AD, numerous investigations have been focused on various configurations (like single- or multiple-stage reactors).

The latest studies considered the breakdown of the AD method into two groups. For example, acetogenesis–methanation and hydrolysis–acidogenesis are accomplished in unconnected reactors, which can enhance the rate of the conversion process of organic matters to CH 4 , although the high prices associated with these types of systems are a critical issue (Yu et al. 2017 ).

More stability and improved efficiency are the outcomes of utilizing multiple-stage bioreactor systems. These types of systems allow for different conditions to be implemented. Obtained results from (Colussi et al. 2013 ) revealed that the two-step AD of corn requires a greater oxygen demand. Marín Pérez et al. (Pérez and Weber 2013 ) stated that the AD physical parting into two phases established the acceptance of various procedure settings for a particular bacteria type, which increased the degradation rate of organic materials. For preventing ammonia inhibition, the two-stage AD of MSW has been implemented (Yabu et al. 2011 ).

A study conducted in 2008 evaluated the one- and two-stage AD in terms of performance. Results showed that the two-stage process had an advanced yield of CH 4 production (Park et al. 2008 ). Figure  6 shows the schematic of multi-stage AD technology.

Standard diagram of a multiple-stage scheme of AD technology

A two-stage AD system has a suitable potential to process a variety of residuals with high microbiological contents. Blonskaja et al. (Blonskaja et al. 2003 ) stated that by using a two-phase AD for distillery waste, a higher rate of methane would be produced. Kim et al. (Kim et al. 2011 ) implemented a four-phase scheme for activated slurry, which allowed extraordinary digestion productivity. The latest improvements in the utilization of molecular biology implements have developed the utility of included microorganisms and the knowledge of the AD practice. Bioindicators and innovative eco-physiological considerations are the ultimate enhancements of the chemical indexes for monitoring and controlling the stability of the AD process (Lebuhn et al. 2014 ). AD process with renewable feedstock has been introduced as a forthcoming method for biogas production. The biogas chiefly consisted of CH 4 (60%) and CO 2 (35–40%). (Abdeshahian et al. 2016 ).

With the aid of the pyrolysis process, pyrolysis gas from biomass resources can be produced. Pyrolysis gas consists of carbon monoxide, hydrogen, carbon dioxide, and extra gases in minor quantities, e.g., methane and some specific components. The biomass resources are lignocellulosic biomass, MSW, lignite, and digestate. The most important advantage of pyrolysis is that the organic components (specifically the relatively dry and gradually biodegradable biomass that is not appropriate for the AD process) can be converted to pyrolysis gas (Luo et al. 2016 ). In the pyrolysis gas production, a methanation process is essential. Traditional catalytic methanation needs high pressure and temperature (230–700 °C) and a metal catalyst, which imposes high cost with low energy efficiency (Guiot et al. 2011 ). Li et al. (Li et al. 2017a ) have investigated the new approach for employing pyrolysis products as a reservoir of carbon for biogas production. In this study, the effects of different parameters on biomethanation of pyrolysis gas have been assessed.

Different strategies, i.e., hydrothermal pretreatment (HTPT), ultrasonic method, alkaline method, and a combination of them, have been used for the dewatering of biomass materials. By considering every aspect of their functions, HTPT has provided the intended benefits (e.g., hot compressed water utilization and decomposing extracellular polymeric substances) (Park et al. 2017 ; Ruiz-Hernando et al. 2015 ).

A prototype for combining hydrothermal pretreatment with pyrolysis and AD process for cogeneration of biogas and biochar has been presented (Li et al. 2018 ). In the hydrothermal pretreatment (HTPT) stage, by heating sludge at 180 °C for half an hour, the water content fell significantly (from 85 to 33%) and dewaterability improved. After that, filtration outputs were subjected to mesophilic AD without any interruption at an approximate temperature of 37 °C. An up-flow anaerobic sludge-bed reactor has been used for biogas production to be consumed in the hydrothermal pretreatment section. Concurrently, for producing heavy biochar, a rotary kiln has been utilized for filter cake pyrolysis at about 600 °C. The considered configuration included a boiler, a pressure filter, a cooling chamber, and a hydrothermal reactor. The sludge was fed into the first reactor (A) and for diluting the sludge, some water was added (20%). In the next reactor, the superheated steam raised the temperature of the sludge (190 °C). By discharging the steam to the first reactor, the pressure in the second reactor decreased (less than 0.11 MPa) and drained steam used for preheating the input sludge (Li et al. 2017b ).

Hübner et.al (Hübner and Mumme 2015 ) proposed a design for biogas production by using aqueous liquor from digestate pyrolysis. In the applied conditions, three main liquors were produced by the pyrolysis process (at 330, 430, and 530 °C) under four chemical oxygen demand (COD) concentrations (3, 6, 12, and 30 g.L −1 ). At 3 g.L −1 , 6 g.L −1 , and 12 g.L −1 a considerable increase in biogas has been observed. Besides, an important feature was that the biogas production in this process did not need any additives.

The studies based on the microbiology field are developing the concept of hydrolytic microbes and biogas production correlation. These types of investigations focused on the hydrolytic microorganisms’ involvement in biogas units, metabolism types, and their functionality in regarded processes. Azman et al. (Azman et al. 2015 ) studied the participation of anaerobic hydrolytic microbes in biogas production from lignocellulosic (by considering microbiological features). Nina Kolesáarová et al. (Kolesárová et al. 2011 ) examined the possibilities for producing biogas with biodiesel by-product as a feedstock in various phases. Yang et al. (Yang et al. 2014 ) presented a membrane gas-permeation for biogas upgrading. In this study, the authors implemented polymer membranes to upgrade biogas production. Furthermore, Miltner et al. (Miltner et al. 2017 ) reviewed innovative technologies in purification and production of biogas. Kiros Hagos et al. (Hagos et al. 2017 ) presented an anaerobic co-digestion (AcoD) process for producing biogas from various diverse biodegradable organic sources. The digestate (fermentation residue) had a high content of moisture that should be dried for increasing the nutrient concentration and decreasing the transported mass. In this case, using a solar greenhouse dryer in tandem with heat recovery from combined heat and power and a microturbine provided a logical opportunity to eradicate the undesirable moisture content. The hybrid case had the potential to reduce moisture content by up to 80% (Maurer and Müller 2019 ). Owing to the faster reaction rates and higher productivity, the thermophilic digestion method is more satisfactory than mesophilic digestion. The mesophilic digestion method leads to a low methane yield and the related biodegradability is relatively poor. On the other hand, these systems represent enhanced stability and higher concentration in bacteria distribution. Unexpected thermal fluctuations affect methanogens performance; as a result, any extreme variation in temperature is undesirable. In this case, it is better to coat the facilities of biogas plants with insulators to control the digester temperature. By building sun-facing biogas units, the effect of cold winds would be eradicated. The integrated system consisted of a solar system and a biogas plant, which provided satisfactory results in gas yield values during cold seasons (Horváth et al. 2016 ).

Therefore, it is reasonable to surmise that biogas production has been influenced by different parameters and factors, including pretreatment processes, feedstock, and additives features, and process technologies. Provided data appear to confirm the following summary of key points.

Production of biogas is an approach for biomass treatment and can help energy generation sustainably. Proper potentials for fossil fuel replacements increased the attention to biogas upgrading and advanced pretreatment methods. The biogas pretreatment procedure has two main steps: 1. biogas cleaning methods and 2. biogas upgrading method. With the help of these strategies, the lignin layer would be broken and the biomass turns to a suitable feedstock for the digestion process, while the porosity increases simultaneously. Hereon, the biogas yield would be improved (based on the feedstock types and associated technologies, obtained rates would be different).

There are different techniques for biogas upgrading that each one has a specific contribution based on the applied commercial technologies. Waster scrubber, chemical scrubber, membrane pressure swing adsorption, and organic physical scrubber contributed the most accounting for 35%, 21%, 20%, 17%, and 5%, respectively.

An extensive variety of compositions has been evaluated and observed for biogas production. Crop biomasses (wheat, barley, etc.), organic wastes (MSW, agro-industry wastewaters, animal manners, etc.), crop residues (wheat straw, barley, or rice straw, etc.), and non-conventional biomass (microalgae or glycerol) fall into this category. Using a wide variety of chemical additives like NaOH, Ca(OH) 2 , NH 4 OH, H 3 PO 4, etc. can improve the biogas production yield. Using additives can improve the AD process stability and lead to up to 40% higher methane yield.

Recent research has been conducted using carbon membranes for biogas upgrading (Lie 2005 ) where the gas separation process was faster. The selected membranes were thin carbon layers with a thickness of less than 1  \(\mu m\) supported on ceramic tubes with a length of 0.50 m. Permeation tests using these membranes showed that the CO 2 molecules permeate 50 times faster than CH 4 molecules. By using such membranes, a typical gas mixture consisted of 0.6 of CH 4 is enriched with CH 4 by only one step separation process to more than 0.9 at 1.20 MPa. The membranes showed excellent mechanical properties after a one-month test. The same membranes are used to separate other gases in the biogas mixture such as H 2 S gas. Such new technologies helped a lot in the biogas industrial process in terms of cost reduction and energy consumption compared to classical technologies such as scrubbing.

As mentioned in this part, process, pretreatment, and feedstock are the main influential parameters for biogas production. For obtaining the highest yield in this term, forming a proper balance between these factors has a significant impact on efficiency. Biomass comprises carbohydrate matters, proteins substances, fats, cellulose, and hemicellulose, which could be employed as raw materials for biogas production. In the existing method, co-substrates improved gas yield by increasing the organic content. Distinctive co-substrates contain organic wastes from agriculture-linked productions, food leftover, and gathered municipal wastes from houses. The composition and yield of biogas production be determined by the feedstock and co-substrate category. Although carbohydrates or proteins demonstrate quicker transformation degrees than fats, it is stated that the second one provides more biogas yield (Achinas et al. 2017 ; Braun 2007 ). To keep away from process non-fulfillments, pretreatment is essential. Employing pretreatment approaches improves the degradation of substrates and then the process productivity. Chemical, thermal, mechanical, or enzymatic procedures can be used to accelerate the decomposition method, while this doesn’t unavoidably affect an advanced yield (Putatunda et al. 2020 ; Mshandete et al. 2006 ).

Policy and framework conditions

The biogas industry expands and develops as it represents an alternative source of energy and has a direct influence on the economy. Worldwide many countries organize the market of biogas through policies and regulations. Well-prepared policies prosper the market of biogas as a renewable energy source.

For instance, by EU policies, instructions, and strategic planning, the portion of renewable energies from 2005 to 2015 increased from 9% up to 16.7%, which is predicted to rise to 20% until 2020 (Irena 2018 ).

Keeping these long-term policies with continuous revision and evaluation is a leader in the world of biogas utilization and marketing (Al Seadi et al. 2000 ; Torrijos 2016 ).

Several organizations and governments such as the European Biogas Association (EBA) and the European Parliament and Council legislated regulations in this regard (Xue et al. 2020 ). The role of such organizations is to prepare new relevant policies for upcoming issues and update existing policies to satisfy the market needs and fluctuations and to harmonize the environment and investment. For example, in the UK there are 118 renewable energy policies compared to 7, 28, and 32 in Denmark, Italy, and German, respectively. This is a common framework, and it is used to write their national policies for organizing renewable energy in Europe. Despite the presence of common European directives such as (2009/28/EC) that considered the biogas production from of agronomic deposits and organic trashes and its application in producing power and heat, several EU nations established their energy markets and biomass sources. These countries issued policies to satisfy their own needs and priorities which known as the National Renewable Energy Action Plan.

China has more than 25 energy policies to manage renewable energy. These policies support the renewable market. Biogas was among these renewable energy sources that benefit from such policies and regulations to develop. The Chinese government governmental parties such as the State Council (SC), the Party Central Committee (PCC), and the Ministry of Agriculture and Rural Affairs (MARA) of China participated extensively in developing policies, regulations, and instruction relevant to the progress of biogas (Gu et al. 2016 ; Hua et al. 2016 ; Wang et al. 2016b ). There is a policy about the development necessities of biogas in rural zones that must be updated every year since 2004 it is called Central Document No. 1 (Ndrc 2017 ).

To point out some Chinese policies, the policy of Measures for the Administration of Rural Biogas Construction National Debt Projects (Trial) was developed in 2003 by its Ministry of Ecology and Environment (MEE) and Rural Biogas Project Construction Fund Management Measures was developed by both MARA and Ministry of Finance (MF) of China in 2007.

Recently in 2019, there has been two policies that resulted in numerous ideas to follow the significant growth of farming and rustic zones, the experimental work program establishing waste-free cities by the state council (SC), and the friendly waste of rustic facilities of biogas production by (MARA). All of these policies regarding the handling, management, utilization, and safe disposal technologies are developed by many authorities in china to avoid the work duplication that might retard the international investment and privatization programs. Clear organization between different authorities helps in generating national priorities and smooth management this includes agricultural activities, finance, trade, and scientific research. For example, introducing a fixed premium subsidy enabled the development of biogas and green gas projects, where a finite budget for a subsidy was determined first. Another country introduced what so-called bioeconomy, especially for such projects. Finally, the harmonized sales tax (HST) is paid on purchases/expenses related to commercial construction and operation of biogas facilities which is also called (input tax credits).

Policy framework

It is important in any policy development to have certain targets to be achieved. These targets are dependent on national priorities, so it is changed from one country to another despite having common targets. Examples of targets can be achieving sustainable development for environment elements, communicate clear standards and regulations for wastes management, environmental laws to regulate the relevant processes and etc. To achieve these targets, action plans revised on a yearly basis to evaluate and update the current regulations for future use.

In general, there are five phases to develop a certain biogas policy, which are:

Phase I creating one regulatory body to coordinate the efforts of all stakeholders who can affect/ be affected by the activities of biogas management, by selecting one focal point to help in decision making and future development. This focal point can be from government or from non-governmental organizations (NGOs). This will unify the efforts to get national priorities and plans.

Phase II developing comprehensive and clear instructions and requirements for biogas production that includes management of raw materials, biogas, safe disposal of biogas wastes, preparing environmental impact assessment (EIA) for current and future facilities, applying the waste hierarchy which is referred to as 5Rs (responsibility, reduce, reuse, recycle, recover), issuing the license and permit to work, and applying the periodic environmental audit.

It is also important to apply the proximity principle for the newly constructed biogas plants and make sure to have a centralized biogas plant where all raw materials from different sources can reach it. The idea behind the one huge centralized biogas plant is to make the audit, monitoring, waste collection, transportation, packaging, labeling, storing and/or and safe disposal as easy as possible and make sure that the best and correct disposal method is applied, for example, in Denmark the Danish Government introduced a total ban on landfilling organic or combustible wastes in 1997 (Al seadi T. 2017 ).

Phase III providing incentives and subsidizing to encourage the facilities to produce biogas and increase its contribution to the economy which is referred to as the green economy and to encourage the partnership with the private sector. Such a program will help the facilities that deal with biogas in rehabilitation activities and waste management. Such incentives and subsidy include tax-free period, free consultation, reduced tariff for raw materials used in the manufacturing procedure, and decrease in the payback period increasing the return on investment of the coming projects which in turn will help in mitigation the biogas sustainability challenges. Contingency plans for unexpected challenges must be considered. The sectors of energy and renewable energy are exposed to many parameters that can affect the energy market such as wars, natural disasters, and nowadays the pandemic of COVID-19 where the prices of oil are dropped drastically (Hübner and Mumme 2015 ) and negative effects are imposed on the industry.

Phase IV providing scientific support to the projects of biogas and waste-to-energy plants. This is necessary to use the best environmental practices (BEP) and the best available techniques (BAT). This can be achieved by technology transfer and scientific research, where each plant must have a research and development (RD) department. Ministry of higher education or any relevant authority in the countries with cooperation with industry can provide funds to universities and plants for more research to utilize the waste in producing energy.

Phase VI participating in international conventions and agreements. Each country must participate in international activities and international conventions relevant to waste management and W-t- E initiatives such as Basel conventions (Basel, 2020) that regulate the transboundary movements of hazardous wastes. This participation is important to make use from the experience of each other and to get the consultation from international experts and to get fund for environmental projects from international agencies such as the German Technical Cooperation Agency (GTZ), Japan International Cooperation Agency (JICA), and United States Agency for International Development (USAID),

Phase VII training and awareness programs, where the concerned parties of W-to-E activities prepare training programs for its staff in the fields of waste management, national and international laws, environmental auditing, risk assessment/management, inspection and licensing. Also, the awareness program for the public is important to educate the people in cleaner production and relevant environmental issues. The role of universities is also important to introduce courses for undergraduate and postgraduate students to raise awareness and support scientific research.

Conclusion and recommendation

With the new applications of biogas, the worldwide biogas industry has increased by more than 90% between the years 2010 and 2018, while further growth is still expected. However, the biogas industry varies significantly in different locations over all the world. Different countries have developed several types of biogas systems which are mainly dependent on different environments as well as on energy demand and supply chain. In this study, the production processes and specific applications of biogas in recent years were reviewed and discussed. In the lack of oxygen, the disintegration of organic material produces biogas that mostly consists of carbon dioxide and methane. In recent years, the exploitation of biogas and the expansion of its potential applications have gained popularity due to factors like climate change, reasonable energy prices, and an increase in distributed generation. Biogas also traditionally known as an off-grid energy resource and can be used in various applications consisting of electricity production and CHP systems. The following key points are summarized from the study:

It is envisioned that the extraction of intrinsic chemical energy of biomass with an efficient AD process can be achieved with proper microbial resource management. Further, advanced monitoring and control of the AD process are needed for the hour for decision making to improve the conversion productivity of the procedure by decreasing the loss of potential methane production due to imbalances of biomass charging rate.

A sustainable circular economy can be created through biomass utilization by recycling organic residues including nutrients in order to bring it back to the society as energy and fuel.

Upgradation of the existing technology for efficient conversion of biomass-based organic residues to biomethane and its utilization as a substitute natural gas or vehicle fuel is the trending research scope.

Hydrogen production using a biogas reforming system with high efficiency is one of the recent applications of biogas. The progress in the application of hydrogen as a clean fuel especially for vehicles is very promising.

Another cutting-edge application of biogas is fuel cells. Recent advances in fuel cells resulting in low emissions (CO 2 , NO x ) and high efficiency make them suitable for power generation and transportation purposes.

Even though the conversion of biomass to biogas through AD has already become a touchable reality in many countries, high financial risks linked to its establishment seek higher financial incentives from the policymakers for sustainable shifting of existing technologies.

Failure of the extraction/utilization of renewable energy sources does not sanction the researchers to explore further, but to transfer any sustainable technology from laboratory to the market seeks ground-breaking effort of the researchers and incentives from the policymakers to handle wisely the transition period of partial/full replacement(s)/modification(s) of the existing technologies/ infrastructures, and social acceptance of the simplified—and perhaps definitive—application of the renewables.

Abbreviations

Anaerobic co-digestion

Anaerobic digestion

Autothermal reforming

Anaerobic membrane bioreactor

Best available techniques

Best environmental practices

Biogas steam reforming

Combined heat and power

Compressed natural gas

Chemical oxygen demand

Coronavirus disease of 2019

European Biogas Association

Expanded granular sludge blanket

Environmental impact assessment

Fuel cell vehicle

Fischer–Tropsch synthesis

Greenhouse gas

Gas turbine

German Technical Cooperation Agency

Hydrothermal pretreatment

Internal combustion engine

International Renewable Energy Agency

Japan international Cooperation agency

Liquefied biogas

Lower-temperature fast pyrolysis

Ministry of Agriculture and Rural Affairs

Membrane bio-reactor

Ministry of Ecology and Environment

Ministry of Finance

National Development and Reform Commission

Non-governmental organizations

Natural gas vehicles

Organic fraction of municipal solid waste

Pressure swing adsorption

Research and development

Renewable natural gas

State council

Solid oxide fuel cells

Thermal phased AD

United States Agency for International Development

Vacuum pressure swing adsorption

Volatile organic compound

World Biogas Association

Waste-to-energy

Wastewater treatment plants

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Abanades, S., Abbaspour, H., Ahmadi, A. et al. A critical review of biogas production and usage with legislations framework across the globe. Int. J. Environ. Sci. Technol. 19 , 3377–3400 (2022). https://doi.org/10.1007/s13762-021-03301-6

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The impact and challenges of sustainable biogas implementation: moving towards a bio-based economy

• Ralph Muvhiiwa 1 ,
• Diane Hildebrandt 1 ,
• Ngonidzashe Chimwani 1 ,
• Lwazi Ngubevana 1 &
• Tonderayi Matambo 1  

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Engineers face increasing pressure to manage and utilize waste (whether of animal, human or municipal origin) in a sustainable way. We suggest that a solution to the problem of organic waste in rural communities lies in their being able to convert it to biogas technology. This would offer smallholders and farmers a long-term, cheap and sustainable energy source that is independent of the national electricity grid. However, although the technology involved in making biogas from waste has already been fully developed, there are obstacles impeding its adoption. First, there is a general ignorance about this source of energy among the very people who can most benefit from using it. Second, at present, South Africa has no regulatory framework to support the installation of biodigesters.

The research focused on the current gap between knowledge and need. The two objectives were raising general awareness of the many and varied benefits that biodigestion can offer, especially to rural communities, and demonstrating how it works. Using science events as a platform, the team introduced the concept of biodigestion, its functioning and uses, to their audiences, and then invited informal responses, which were recorded. The second stage, the case study, entailed the setting up of a small-scale (10 m 3 ) household biodigester in the Muldersdrift community in Gauteng, South Africa. It was put into operation, using fresh cow dung as the feed. Members of the community were invited to watch every step of the process and afterwards were asked to participate in a more formal survey, which sought their opinions on whether biodigestion offers a power source the individual farmer could (and would) use.

The results presented in this paper were derived from a comparison of the ‘before-and-after-installation’ responses of the persons interviewed. We found that the members of the Muldersdrift community who had been involved in both phases of the case study (explanation followed by experience of a hands-on educational example) had become more willing to adopt the technology.

Conclusions

The results justified our contention that, to ensure a greater adoption of biogas technology in South Africa, it is necessary to provide targeted communities with educational programmes and exposure to pilot plants.

Although conversion of waste to biogas is an established technology, it has been under-used, probably because until recently, electricity was relatively affordable to most of the population in South Africa. However, the increase in both the demand for, and the cost of, electricity has prompted engineers to revive their interest in rolling out biogas technology in South Africa [ 1 ].

A salient reason for advocating biodigestion as an alternative source of energy is that electricity is not available in all parts of this country. Approximately 2.328 million households, estimated by Triebel and Damm [ 2 ] as representing 25–30% of South African families, meet their energy needs with traditional fuels such as firewood and charcoal. Most of this group live in deprived circumstances in the urban slums and rural areas but have no knowledge of biogas technology.

The overarching purpose of the Materials and Process Synthesis (MaPS) team’s research during this project was to test the hypothesis that if biogas can be fully exploited, it can supply a means of overcoming energy poverty in rural South Africa.

The authors reasoned that in order to introduce biogas to South Africans who lack access to a power supply, the first requirement must be to introduce them to the concept of biogas, and the second to teach them how the technology works. They started the process by identifying rural communities that did not have access to electricity, with a view to introducing them to the nature and functioning of biodigesters. A collaborative approach was applied throughout, and community members were asked for their views and queries about biodigestion both before and after the pilot digester had been commissioned.

It is worth noting that our project was in alignment with South Africa’s Development Plan (NDP) and bio-economy strategy, which aim to promote bio-innovations to achieve a sustainable economy based on biological resources, materials and processes. The production of biogas is also synchronous with the United Nations Sustainable Development Goals (SDG), especially goal numbers 1 and 7, which require that this technology can help reduce socio-economic poverty by providing clean energy that from renewable sources. South Africa is also signatory to the Kyoto protocol, which undertakes to cut greenhouse gas emissions back by 34% by 2020 and 45% by 2030 [ 3 ].

Biogas as an energy solution to rural South African communities

Due to the current energy shortages and cost of raising capital in South Africa, it is likely that the national power supply company (ESKOM) will be unable to continue expanding its network into the rural areas. This has given impetus to the search for alternative energy sources. Biogas offers a cheap, renewable and viable solution to the problem of providing energy to rural communities and farmers [ 4 ] and also has the merit of using waste that has been traditionally regarded as useless, as the feedstock.

The technology involved in biogas production is fairly simple and can be implemented cheaply and efficiently by means of small-scale digesters that are easy to use and maintain. These household biodigesters can offer benefits to all spheres of society but have a particular bearing on the needs of farmers in rural areas. They can use the gas produced for cooking and lighting, for charging batteries from running biogas generators, and for fertilizing crops with the residual waste.

Another reason for identifying this group as most suitable for putting the biodigestion technology into practice is that small farmers generally have free access to livestock waste, which provides feedstock for the digester. Normally, rural households use the raw manure obtained from their animals as a form of plant fertilizer, but this has a lower organic nitrogen content than the slurry created by the biogas digestion process [ 5 , 6 , 7 ], which is odourless, and makes a better fertilizer.

Also, the combustion of biogas provides a clean source of energy, as it does not produce soot, like firewood. This helps reduce indoor air pollution, which in turn prevents respiratory infections and associated diseases [ 8 ]. According to an evaluation by Pal [ 9 ] in India, a biogas digester producing 2 m 3 of biogas per day can replace approximately 270–300 kg of firewood per month, depending on the quality of the biogas. Studies of the domestic use of biogas carried out in rural areas in Zimbabwe and Kenya [ 10 , 11 ] also found that using biogas for cooking was more time-efficient than conventional fuels, and this was a key factor in the willingness of people to adopt it. Although time is required to collect waste and feed the digester, it is a much shorter period than the equivalent required to gather firewood and charcoal.

Perhaps, the most important of its many advantages is that biogas can offer a decentralized energy solution to rural communities in South Africa.

The barriers to expansion and acceptance of biogas production in South Africa

There are currently around 700 biodigesters in South Africa [ 4 ]. About 50% of these are small-scale domestic digesters, and only 10% are commercial installations [ 4 ]. The remaining numbers, representing approximately 40% are installed at wastewater treatment plants. There is still much room for further expansion, but various difficulties, not connected with the technology as such, impede it. The political and regulatory aspects of making access to biodigestion possible in South Africa are discussed briefly in a later section.

The focus of this article is on a key issue discussed during the National Biogas Conference, hosted by the Southern African Biogas Industry Association (SABIA) on 5 March 2015: the lack of awareness and understanding of biogas as a form of energy in the general public, which hinders the expansion of this technology in this country [ 12 ]. It was this point on which the project carried out was based.

Currently, most people who have the raw materials readily available do not have any knowledge of biogas technology. It is therefore important to educate them by first explaining and then demonstrating this technology to rural communities. This would allow the team to deal with some misconceptions that smallholders and farmers might have about biogas, increase their understanding of the technology, and consequently enable them to realize the benefits offered by it. Their acceptance of its usefulness is essential to their willingness to adopt biogas as a source of energy.

It is very difficult to devise a strategy with which to approach communities if the promoters have no understanding of the pre-perceptions and concerns of the farmers themselves. For similar reasons, the uptake of biodigesters in other African countries is not high. In Kenya, biogas technology is not new, but the adoption process is still slow, owing to inadequate funds, poor infrastructure and a general ignorance of this technology among the people who might derive the greatest benefit from it [ 11 ].

For all of the above reasons, the project designed by our team entailed two steps that would enable us to understand better how to increase the acceptance of biodigester technology within rural communities in South Africa. The first was to establish the level of knowledge about biogas technology in schools and rural areas. The second was to examine the differences in the views and responses of members of a rural community after the installation of a biodigester in their vicinity. A small-scale bag digester (approximately 10 m 3 in volume was set up in the Muldersdrift community by a team from Engineers without Borders, based at the University of South Africa (EWB-Unisa). The feedstock for the biodigester was fresh cow dung. The performance of the biodigester was rated according to the typical energy requirements of a household, such as gas cooking, lighting, and heating water.

Survey methodology

The first, informal survey followed a qualitative approach because the team wanted to gather information on which they could base and interpret the quantitative approach used in the second survey [ 13 , 14 , 15 , 16 ]. The first was based on the spontaneous responses of participants in the science conferences to the concept [ 17 ]. (This type of approach allows the researcher to focus his or her efforts on gathering rich data from answers to the research questions). The second, more formal survey focused on the actual experience of smallholders and farmers witnessing the installation of the biodigester, the way it worked, and their assessment of its utility.

Step 1: Survey of perceptions concerning biogas technology

In order to understand the level of knowledge communities in South Africa have on biodigester technologies, the writers took part in four of the country’s biggest science events, including “The Science Festival Africa” in Grahamstown and “The Sasol Expo” in Sasolburg, within the space of 2 years (2015–2016). The conferences were mainly held in small towns, located in predominantly rural and agricultural areas. The exception was Sasolburg, which is part of the so-called Vaal Triangle, which is highly industrialized, but is surrounded by agricultural land. Many people, largely comprising school pupils, members of the surrounding farmers, attended these events. At all of these, a simple cardboard model of a biodigester (shown in Fig.  1 ) was used to introduce those present to the nature and function of biogas and to invite their feedback. The purpose was to elicit what knowledge they had of biogas, their perceptions concerning it, and how safe they thought it was. There was no set questionnaire: it was an informal survey to determine people’s responses to the idea of biogas. The researchers were particularly interested in finding out whether they were aware of biogas technology and its uses, and, if so, what level of understanding they had achieved. For those who had never come across the concept, we questioned them to gauge their reactions to it. Our central objective was to find out whether, or under what circumstances, they would embrace the idea of biodigestion. This approach is also very similar to the process synthesis approach used in chemical engineering to identify the most important factors in a complex system relatively quickly [ 18 ].

Simple display used by EWB-Unisa at the Science Expos to discuss biogas technology

While step one involved ascertaining the knowledge of biogas technology in attendees at the science events, step two entailed a practical demonstration of how the technology works. The team did this by involving some members of the local farming community in the building and commissioning of a pilot biodigester.

Step 2: Case study—implementing biogas technology in a rural/farming South African community

Case study location.

The researchers from the team had met a small-scale farmer in Mulderdrift when they were looking for manure for their laboratory experiments on anaerobic digestion to produce biogas. He had shown interest in what they were doing, as he had never heard of biogas before. Over time, he became familiar with the team and the work they were doing and was very enthusiastic about seeing how a digester, built on his land, would work in practice.

A small-scale biodigester was built by the EWB team on a small farm in Muldersdrift, on the outskirts of Johannesburg. The area surrounding the location of the biogas plant comprises both agricultural plots and farms, and it was from a catchment area within an approximately 3-km radius of the plot on which the digester was to be built that we recruited local people, mainly farmers and farm workers, who were willing to participate in our study. No specific criterion was used to pick those surveyed, and the general answers we were given by the respondents were obtained through informal conversations/purposive sampling. These are shown in a later section of this research paper, when the “before” and “after” stages of the surveys are compared.

The design and implementation of the biodigester

The biodigester chosen for the Muldersdrift experiment was of the biobag variety because it is more easy to maintain than a fixed dome brick digester. The design used a large biobag (made of durable reinforced and bacteria-resistant polyvinyl chloride (PVC) which can have a lifespan of more than 15 years). The biodigester is 8 m long and has a diameter of 1 m. Two manholes were constructed, using cement bricks to form the biodigester inlet and outlet. The digester (PVC) bag was then connected to the inlet and outlet using 25-cm diameter PVC piping. The biodigester bag was placed in a trench that slopes slightly downward from inlet to outlet so that the inlet pipe was placed 20 cm higher than the outlet pipe. This was necessary for two reasons: the biodigester operates by means of gravity displacement; and also the difference in height forms a liquid seal preventing air from entering through the inlet pipe into the biodigester. This system is simple to construct, when compared with the conventional dome-shaped biodigester design. Another advantage of this type of biodigester is that the actual digester is made from light-weight PVC plastic, and the only major construction effort required is digging the manholes.

Once a digester has been installed, fresh animal dung is collected and mixed with water in a ratio of at least 1:4 by volume to form slurry. The cow dung is collected from a cattle kraal where the animals sleep at night, but graze on a free range paddock during the day. (Incidentally, a drawback of the biogas process is that it requires a lot of water.) Twenty litres of slurry is fed every 2 days to the biodigester. The digestion retention time is around 20–40 days, during which time the waste material is broken down by a consortium of the bacteria that occur naturally in the manure, to produce biogas (mainly methane and carbon dioxide) in the absence of oxygen. As the waste begins to digest, the biogas produced inflates the biobag, and the gas is released through a valve to piping that is connected to the appliances in the farmhouse. The gas passes through a pressure pump (alternatively some weights, typically old tyres, are placed on the biobag to build up a pressure of 2.5 KPa, which is the minimum needed by the biogas stove or lamp). The gas pipeline also passes through a moisture trap and a desulphurising unit, which can be made by using a container filled with iron filings. All these small units are needed before the gas can be used, but all are cheap to manufacture. The whole process is presented in Fig.  2 , and a picture of the inflated bag is shown in Fig.  3 .

Typical small-scale biodigester system for rural operation

Typical operational biodigester at the small-scale farm in Muldersdrift, Johannesburg, South Africa

The digester used was supplied by Biogas SA and (as already noted) is simple to install and operate. The cost of a full kit imported biobag was about ZAR16 000 (USD $1120) in 2015/2016. Although this is a once-off cost, this amount is beyond the reach of many rural households. However, bricks can be moulded locally and if cement can be obtained, a cheaper type of digester can be built for a small household.

Performance of the biodigester

The biobag has a gas volume of about 4–5 m 3 , and its output comprises around 53% of methane gas concentrate and the remaining 47% of carbon dioxide available for use per day in summer, when the temperatures average 25–30 °C. This gas can be used for a cooking for 2–3 h a day and provides about two plus hours a day of lighting (using a gas lamp). The farmer can also use the gas to run a 700 W biogas generator for an hour per day (he can use this for battery charging) as well as to heat water for bathing in a 7-l/min gas geyser. The amount of water heated was sufficient for the use of three adults. The digester is fed with a 20 l amount of fresh waste slurry every second day, which is enough to supply the gas requirements of the household. The use of biogas saves the farmer about 1 h per day, as he no longer has to spend time fetching and preparing firewood.

In winter, when temperatures are low (~15 °C), the range of usage becomes more limited, as the digester produces only enough gas for cooking. No gas is produced when there is frost (<10 °C) because the activity of the methanogen (mesophiles) bacteria reduces with the drop in temperature and becomes completely inactive at temperatures lower than 10 °C [ 19 ]. The waste that is fed into the biobag during the coldest months can take up to 30 days to digest and produce gas so that the waste that the farmer feeds in today will produce usable gas only in about a month’s time.

Community survey after exposure to biogas technology

Throughout the step-by-step construction and operation of the digester, we explained to the community members how the production of the biogas gas takes place, and the different ways in which it can be used. We made clear that although bacterial activity helped to produce the gas during the decomposition of waste materials, the resultant gas did not contain dangerous bacteria. After this two-phase introduction had been completed, most of the participants indicated that they would adopt the technology if it would save them more time and money than relying on traditional sources of energy.

The post-installation survey took the form of a qualitative, cross-sectional study with purposive sampling. The research data were obtained from about 25 people and took the form of a questionnaire that aimed to assess whether they had a basic knowledge of science, what they knew about biodigesters, and their attitude towards biodigester technology and science in general. None of the respondents to this survey had visited any of the Science Expos. We also looked at demographics concerning race, age and their rating according to the Living Standards Measure (LSM), which is commonly used in South Africa. Some of the questions concerned the energy sources currently used by each respondent and the problems connected with employing them. One of the key questions asked concerned the respondent’s access to feed, water and the transportation of biodigester feed. It was also important to establish the main income-generating activities of each participant, as well as his or her ability to maintain the digester. Other questions involved the capability of the participant to adhere to the safety regulations for biogas use and his or her keenness to learn more about the technology.

Results of surveys from Science Expos

The survey conducted at the science events revealed that less than 10% of the high school pupils interviewed had any knowledge of biogas technology. What was also very surprising was their resistance to the concept. Most students said that the technology was not possible and also not “ethical”. These students were concerned about the source of the biogas. They thought that since it is made from manure (or worse, sewage), it could be contaminated, and use of the biogas could cause illness. The local farmers surveyed also appeared to have little knowledge of the biogas technology, but in contrast, they were very willing to learn and implement it if it offered any benefit to them. An important aspect for the farmers was their wish to see a working biodigester unit and also to have a clear understanding of the economics concerned before they implemented the technology. At the science event in Sasolburg 2016, students from 7 out of 46 schools survey had knowledge of biogas. Although Sasolburg is industrialized, and the community in the area seemed to have some awareness of biogas, they still lacked information about it and had had no exposure to this technology.

Results of the Muldersdrift case study

Initially, 64% of the respondents including the owner of the farm on which the biodigester was located had no knowledge about the nature and application of biogas while the balance knew about biogas. The remainder had seen programmes on the subject on television, while others had read about it.

It was clear from the data that there is also lack of proper structures that can help to inform people about the advantages of using biogas because the majority of respondents to the first survey had no knowledge of this technology. This is especially true in rural areas where people have access to the required waste materials.

Also, information on biogas is mostly available only to researchers, not the potential users of this technology. This poses the need for us to bridge the process of turning the technology and research into practice in South Africa. The opinion expressed by SABIA on this issue is that in order to boost the public’s awareness and understanding of the biogas technology in South Africa, a great deal of financial support will be required [ 12 ]. This has not so far been forthcoming.

The case study found that after the educational process, most community participants indicated that they would adopt the technology if it would save them more time and money than the sources of energy currently available to them. Our analysis of the case study results showed that there were a number of key themes that emerged from the answers of most of the respondents.

Community survey pre-installation

These themes are discussed individually below and are based on pre- and post-implementation responses.

Feedstock availability

It was clear from the survey data that there was a pervasive perception that biogas technology works only for people who have sufficient animal and agricultural waste available to them to obtain a reasonable quantity of gas for energy. This perception is definitely not unfounded. However, crop production and animal husbandry are predominant in the rural areas and farms, so this section of the population can find ways to tap into this supply of material. About 68% of respondents, many of them, farm labourers or workers in the area, mentioned that although they did not own livestock, they were willing to travel to collect animal waste from neighbouring farms.

Hygiene-related concerns

Many of those surveyed expressed concern that biodigestion is unhygienic, as one uses smelly and bacteria-infested waste to produce energy. Although the pre-implementation data was not quantitatively recorded, more than 50% of the people surveyed after the science events expressed worry and doubt that biogas was hygienically acceptable. They feared that they would be exposed to contact with the (dangerous) bacteria in the waste material, for example while preparing the slurry feed for the digester, or during cooking, and that this would be harmful to their health.

However, after the installation, most of the people who had seen how the technology worked had accepted that it posed no health risk. Figure  4 shows that 88% of the respondents do not have any concerns about the hygiene of using biogas technology for cooking purposes. Although these data do not allow an accurate comparison of pre- and post-perceptions of the risks of changing to biogas, the researchers were confident that they had seen a positive change in mindset.

Post-implementation: hygienic perceptions about the use of biogas post-implementation

Access to energy

Another prevalent theme raised in the pre-implementation survey was that community members are accustomed to using traditional energy sources like coal, paraffin and firewood and that they see no reason why they should shift to biogas.

The survey, as presented in Fig.  5 , shows that 80% of the people in the community use electricity and paraffin as a source of energy. The owner of the farm where the digester was installed had used firewood, electricity, paraffin and LPG interchangeably for cooking. Currently, after the installation of the biodigester at the farm, 100% of the farmer’s energy supply for cooking comes from biogas, except in winter when gas production is low. Post-implementation, after seeing how the technology works, 92% of the respondents indicated that they would be willing to change from their traditional sources of energy to biogas for cooking (Fig.  6 ). Only one person indicated a refusal to change to biogas, preferring to continue using his current source.

Post-implementation: sources of energy currently used

Post-implementation: biogas adoption in relation to performance of technology

A matter raised by most of the interviewees was that, while some of them see this technology as not practical, others are constrained from trying it only by a lack of the necessary building materials and knowledge of how to implement the technology, and, most particularly, the means of meeting the capital costs of building such a structure.

The capital cost involved in purchasing and installing a biodigester presented a major challenge to the interviewees, although 92% of the respondents to the formal survey indicated that they were willing to adopt this technology. One respondent expressed a willingness to provide 50% of the capital cost, while another could raise only about 7.5% of the amount needed. None of them envisaged that the cost of maintaining the digester would present a problem.

Safety and emissions

Although the nature of biogas technology raises the possibility that a biodigester represents an explosion risk, generally biogas is a safe technology. The concentration of methane ranges from 40 to 70%, which is low compared to the concentration in LPG gas (90%≥). The biogas inside a biodigester is usually at an operating pressure of around 2.5 KPa, low enough to avoid an explosion. If the biogas leaks from a small biodigester, the gas can become relatively diluted by the ambient air, as the biodigester is typically constructed in the open. The building/kitchen where the biogas is used needs to be well ventilated so that in the event of a leak, the gas is diluted. It is also assumed that if the user is able to follow the safety measures for using LPG gas, then he or she should be able to adopt and use biogas. The process is anaerobic, meaning that it occurs in the absence of oxygen; thus, as long as the pressure in the digester is higher than atmospheric pressure, the chances of an explosion are reduced as oxygen is required for combustion. In general, biogas is lighter than air and hence escapes into the atmosphere in the event of any leaks; whereas, even small leaks of LPG gas, which is heavier than air, can lead to an explosion. After practical demonstration through the building of the biodigester, 84% of the respondents to the survey perceived the handling and use of biogas technology as easy and safe. The relevant appraisals are summarized in Fig.  7 .

Post-implementation: handling of biogas after exposure

In addition, communities need to be educated regarding the fact that 99% of the pathogens and bacteria in the feed are destroyed in the digester under anaerobic conditions, making it safe to handle the bio-slurry (also referred to as bio-fertilizer), which can be used for vegetable farming. Furthermore, the biogas is effectively bacteria-free and is thus safe to use. The smell that may come from the gas comes from sulphur-containing compounds and can be controlled by passing the gas through iron filings, leaving an odour-free, clean-burning fuel. However, in the pilot demonstration, the farmer reported that there was no odour from the manure nor were any operational problems connected with the entire process.

Age group and profession survey

Figure  8a , b compares the different ages of the people surveyed, and the nature of the work they do in the community. Sixty percent are workers, in jobs ranging from self-employed, farm or lodge employees, or people doing piece jobs. Twenty percent are farm owners, and the remaining 20% are unemployed. Thirty-one percent of the respondents were aged between 31 and 40 years while 50% were above 40 years old.

Post-implementation. a Age grouping of the respondents. b General occupations of the respondents

Summary of results

Overall, the survey indicated that two issues remained problematic.

Feedstock availability: Many respondents felt that only those with access to fairly large numbers of feedstock whose numbers remain consistent can benefit from this technology. Consequently, biodigestion was seen to be most suitable for farmers or farm workers.

Cost: The majority continued to believe that the capital costs of installing a biodigester are too high. This may be a challenge that needs to be addressed by government and entrepreneurs. For example if biobag biodigesters could be manufactured in South Africa, that might drive down the cost to more affordable levels.

These responses were similar to those found by Matsvange [ 10 ], who carried out research on changing to biodigestionat different locations in Zimbabwe. The findings were that people are willing to adopt the technology if the questions of availability of feedstock and cost were addressed.

Based on surveys and the answers of the respondents, our research team has elicited that education and ‘hands-on’ exposure to biodigestion have a positive effect on the adoption process. A clearer understanding of biogas technology will impart greater confidence in potential users, which will increase the likelihood that the technology will be adopted. This in turn will be supported by noticeable benefits, as suggested by many other researchers. These include less time spent on gathering energy sources for cooking, thereby freeing up time for other activities; reduction in deforestation, a cleaner cooking process.

The benefits of lighting, cooking and time efficiency mentioned in the literature were actually demonstrated in our research by the reports of the recipient of the biodigester. Although research has also shown that the initial (and most important) barrier to adoption of biogas technology is lack of knowledge; other constraints emerged once that knowledge had been imparted.

The barrier of capital costs is formidable: researchers and government need to work together to make cheaper digesters available and to supply financial support to enable households or communities to build biodigesters. The energy consumed by the two-plate stove at Muldersdrift farm was rated at 2000 W. It was used on average for 2 h per day for 30 days a month. The price of electricity, as set out in the electricity tariffs for the 2014/15 Mogale City Local Municipality, was ZAR 1.5423/kW h [ 20 ]. The total amount saved by changing to biogas per year is ZAR2 220.91 (approximately USD $155, using an exchange rate of 1 USD: 14.34 ZAR—December 2016). To extrapolate, after 15 years, an amount of ZAR33.313.68 (USD $2325) can be saved, using as a template the cost of cooking by electricity. More savings can be achieved if the calculations include power for lighting and geyser and generator usage. Although the capital cost of the imported biobag kit is a once-off amount of ZAR16 000 (USD $1120), and the construction costs are around ZAR 5000 (USD $350), the use of biogas has a long-term cost benefit, as the analysis of cost saved shows. This cost benefit will be enhanced if the bag can be produced more cheaply locally, or if less profit is made on the sale. The results of this case study show that if biogas use was adopted on a large scale, a sustainable bio-based economy is attainable.

Although this study was carried out in one location, which may limit its general applicability, it was clear from the post-implementation survey in the Muldersdrift community that there had been a complete shift in attitude after the local farmers had seen a biodigester constructed and put to work. This then suggests there is a need to roll out more digesters in similar rural societies.

However, this initiative faces barriers other than acceptance of biogas digestion by the targeted communities. The introduction of a new technology requires policy support from South Africa’s government, which itself needs to understand how biodigestion works, and what potential it has to improve the lives of ordinary Africans. To date, there is very little, or no, information available on how much the country’s decision-makers and even average South Africans know about biogas, or biogas technology. Unless provision is made to educate both the authorities and the public on the advantages of changing to biogas and to demonstrate that biogas technologies are safe and secure, there can be little hope that the necessary policy framework and start-up financing will be provided by the government. This would probably involve training a number of facilitators who can help the public to become aware of and assimilate the working and nature of biogas production.

At present, despite intensive planning and the efforts made by the Department of Trade and Industry (DTI) and other stakeholders, we continue to lack an adequate regulatory structure to support a large-scale launch of biogas technology [ 12 ]. According to the Department of Energy, the owner of a biogas project is required to register with the National Energy Regulator of South Africa (NERSA), which requires that the owner conform with multiple environmental regulations. These in turn have resulted in complex zoning legislation that must be complied with before any waste to energy biogas projects can be initiated [ 21 ]. At present, South Africa has no legal or policy guidelines to facilitate registration with NERSA, or to simplify compliance with zoning regulations. Another obstacle is the intricate administrative processes currently needed for project development and authorisation, especially at municipal level.

In order to tackle these issues, various stakeholders, including the Department of Energy (DEO), have begun to draft a policy framework for the installation of biodigesters in remote regions of South Africa and to identify rural households that would be able to use one. Accordingly, the framers of the policy should aim to take into account the availability of suitable feed, water and finance in the case of each recipient, as the biodigester should be sustainable in terms of cost. The last is of vital importance. At present, South Africa’s government does not make any provision to fund, or create, dedicated financial mechanisms, incentives and grants to assist the adoption of biogas. The most serious obstacle to supplying digesters to the rural poor is the capital outlay required to buy and install them.

Some progress has been made. Currently, a committee is being set up to consult on legal issues relating to the registration, certification and licencing of rural biodigesters. Yet despite the advances made in policy in recent years, there remains a gap that needs to be filled.

This research has shown that education and exposure are the key tools required to help increase the adoption of biogas in rural and small-scale farming areas. The judicious use of these tools (education and exposure) could help unlock the enormous promise that we can build a bio-based economy, in by these means alleviate poverty in rural South Africa, both as far as energy provision and a better standard of living are concerned. The findings also show that a successful collaboration between research and community engagement can generate knowledge and skills that can be transferred to help a community to adopt biogas as a form of renewable energy. We also recommend that government should play a role in disseminating biogas technology as a renewable source of power in rural areas. This would help to promote greater awareness of the technology, which in turn would expand its adoption. The construction of pilot digesters in rural communities will also expose the members of that community to the practical advantages of this technology, and thus help them to enjoy its benefits.

It is important that policy makers should note that education is the driving force because it can erase misconceptions. There is therefore a need for the government to provide platforms for learning and demonstration of biogas technology in order to support and expand the application of this sustainable form of energy.

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Acknowledgements

The authors would like to thank The University of South Africa’s Community Engagement Department for the financial support that made this research possible. They also express gratitude to the Materials and Process Synthesis Research Unit and Engineers Without Borders, South Africa for the technical support given during the research and the implementation of the projects. Also special thanks are due to the UNISA student members who contributed their knowledge and sacrificed time to construct the digesters, which made it possible for us to collect the information contained in this paper.

Authors’ contributions

RFM was involved in installing the biodigester and carrying out the social research, and he was involved in the writing and editing of the paper. DH was responsible for writing and editing the research work, and she is also the project leader for this research work. LN was responsible for writing and editing the research work, and he was hands on during the conceptualisation of the research project. TM was responsible for writing and editing the research work. NC was responsible for carrying out the actual research work as well as research writing and editing. All authors read and approved the final manuscript.

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Ralph Farai Muvhiiwa is a chemical engineer by profession and is the Chairperson for Engineers Without Borders at the University of South Africa. He heads the community engagement flagship by trying to put engineering knowledge into practice.

Diane Hildebrandt is a professor in Chemical engineering.

Lwazi Ngubevana has a doctorate in Chemical engineering.

Tonderayi Matambo has a doctorate in Biotechnology.

Ngonidzashe Chimwani has a doctorate in Chemical engineering. He is currently an active member of Engineers Without Borders.

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The authors declare that they have no competing interests.

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Muvhiiwa, R., Hildebrandt, D., Chimwani, N. et al. The impact and challenges of sustainable biogas implementation: moving towards a bio-based economy. Energ Sustain Soc 7 , 20 (2017). https://doi.org/10.1186/s13705-017-0122-3

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How to use biogas?: A systematic review of biogas utilization pathways and business models

• Anica Mertins   ORCID: orcid.org/0000-0003-1462-6875 1 &
• Tim Wawer 2  

Bioresources and Bioprocessing volume  9 , Article number:  59 ( 2022 ) Cite this article

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There are many options for the utilization of biogas in different energy sectors (power, heat, mobility). The technical possibilities of using biogas are more diverse than the actual business models applied in the biogas industry. This paper shows the possible utilization pathways of biogas, divided into coupled power and heat generation, direct utilization and upgrading to a gas of a higher value. Subsequently, an overview of the business models discussed is given by a systematic literature review. The latter shows that the investigation of biogas business models is focused mainly on the last decade and has increased slightly over time. The regions of investigation can be found worldwide, with a clear focus on Europe. Direct use is studied mainly in the Asian and African regions. In the European context, a shift from investigating combined heat and power use to upgrading the biogas produced is evident.

Introduction

Over 90% of the biogas produced in the world was used for the production of power and heat in 2018, with only the remaining 9% being used as biomethane in the mobility sector or for injection into the natural gas grid (International Energy Agency 2020 ). However, the increasing number of biomethane plants suggests that the share of usage as biomethane will increase in the future (Banja et al. 2019 ). The utilization pathways differ between the countries, depending very much on the framework conditions of the country in which the biogas plant is operated (Capodaglio et al. 2016 ). Biogas plants in different countries have developed differently in terms of the substrate usage, pretreatment technology, plant size and utilization pathway of the biogas (Stürmer et al. 2021a ). Several regions throughout the world are striving for climate neutrality based on an energy system supplied 100% from renewable energies. This means that not only the power supply, but also the heat and fuel supply have to abandon fossil fuels. This raises the question what the future will look like and what role biogas technology will play in it.

In this paper, the utilization pathway is defined as a technical possibility for the use of biogas. This includes particularly converting biogas into usable energy and the subsequent possible fields of application. The term ‘business model’ is used to describe the options for monetizing biogas. In addition to the revenue streams, the business model specifies the utilization pathway and the profitability in a sales market, including costs.

This paper presents a systematic review of utilization pathways for biogas and evaluates the business models associated with each pathway. Conclusions can be drawn about the temporal course of research interest and geographic characteristics. The various business models that are conceivable based on the utilization pathways are presented. This approach reveals differences between the technical feasibility of a usage pathway and its actual implementation as part of a business model. These business models are examined independently of any specific region and should, therefore, be considered independently of local funding programs.

Several publications have already examined and compared different pathways of biogas, for example (Bystricky et al. 2010 ; Patrizio et al. 2015 ; Wu et al. 2016 ). Exemplarily, the use of biogas in a decentralized combined heat and power (CHP) plant, the injection of upgraded biomethane into the gas grid for off-site power and heat generation, and the use of biomethane as fuel are all under investigation. Business models for biogas have also already been investigated in the literature (Heffels et al. 2012 ; Horschig et al. 2019 ; Karlsson 2019 ). Focuses are, for example, on the direct marketing of power or different possibilities for biomethane becoming advantageous as a business model. To the best of our knowledge, no comprehensive analysis of biogas business models in the different pathways exists in the current literature. The novelty of this paper is, therefore, the analysis of the technical possibilities for the use of biogas combined with a detailed presentation of possible business models that are discussed in the literature. The specifics of each business model are discussed and the research strength in the area of the different business models is shown. In the context of the transformation of the energy system, it is relevant to reveal technical possibilities for energy generation that enable a profitability. The connection of technical utilization pathways with the consideration of possible business models plays a central role in the design of future energy supply, such as the use of biogas.

The paper is structured as follows: firstly, an overview of existing technical possibilities for biogas utilization is given. Subsequently, the method of literature review is presented. The results of the literature research on biogas business models are then presented in detail, subdivided according to the different utilization routes. The discussion of the results is followed by a conclusion.

Biogas utilization pathways

Biogas is a renewable energy that can be used in various ways, which is a major advantage of the technology. Possible utilization pathways are conceivable in all areas of consumption (Capodaglio et al. 2016 ), with three main distinctions:

Using the biogas in a CHP unit;

Using the biogas directly, for example, in machines or facilities in agricultural operations; and

Upgrading the biogas to a gas of a higher value.

The different pathways are shown in Fig.  1 and are explained in detail below.

Potential utilization pathways for biogas

Pathway 1: CHP generation

The most commonly used pathway for biogas is in decentralized CHP units. The biogas produced is first dried, desulfurized and then used in a gas engine that drives a generator delivering power and heat. The power is normally used externally and, therefore, fed into the grid. Part of the generated heat is needed to operate the digester. The remaining heat can be used externally, for example, to heat buildings or for crop drying (Watter 2019 ). The electrical efficiency is usually between 30 and 40%, and the corresponding thermal efficiency is 35 to 55% (Lantz 2012 ). The highest mean fuel efficiency is achieved when all the heat produced can be used all year round (Fischer et al. 2016 ; Watter 2019 ).

A CHP unit can be operated in a base load mode, i.e., generates a constant amount of power and heat, or in a flexible mode. The latter enables the energy generation to be adapted to a demand for power or heat, or is driven by price signals. The digester is designed for a constant output, therefore, the flexibilization requires the adaptation of components (Liebetrau et al. 2015 ).

Regarding existing biogas plants, which were originally designed for base load operation, two retrofitting options for flexible generation are available: they can be adapted for flexible power production either by lowering the rated output or by increasing the power generation capacity. For the latter, a second CHP unit, a larger gas storage tank, facilities for external heat utilization and an adaptation of the connection to the power grid (e.g., transformer) are required. Alternative to the expansion of the gas storage, flexible biogas production can be achieved by changing the feedstock-feeding management (Daniel-Gromke et al. 2019 ).

One advantage of on-site use of a CHP unit is that the energy generated can be used to cover the internal energy demand of the anaerobic digestion process. The power demand for the digester and ancillary components in other utilization pathways must be covered from the power grid. Heat, on the other hand, can be generated by a gas boiler (Goulding and Power 2013 ).

Pathway 2: direct usage of biogas

In the second pathway, biogas is used in the immediate vicinity of the biogas plant. The aim of the on-site use of biogas is to reduce energy procurement costs or to reach energy self-sufficiency. Biogas can be used directly in households (Hamid and Blanchard 2018 ), for uncoupled heat generation, e.g., in heat boilers for use as process heat or steam or as fuel for machines on the farm (Lampinen 2004 ).

Another option is the direct utilization of biogas in tractors or agricultural machinery. Currently, biogas is not used in common practice for mobile internal combustion engines without upgrading (Owczuk et al. 2019 ). As untreated biogas does not meet the minimum specifications of a compressed natural gas (CNG) fuel due to the comparatively high proportion of carbon dioxide (CO 2 ), water vapor and hydrogen sulfide, it cannot be used in CNG-powered vehicles (Kruczyński et al. 2013 ). Compared to biomethane or natural gas, the energy content of biogas per unit mass is significantly lower. Large quantities of biogas can, thus, only be stored under high pressure, with the upgrading, storage and handling in this case leading to increased costs (Mihic 2004 ). On the other hand, biogas that has been separated from hydrogen sulfide and water is a significantly cheaper fuel than biomethane (Kruczyński et al. 2013 ). Another difficulty of using pure biogas as a fuel is to ensure a secure supply. Thus, periods with low biogas production or high consumption must be balanced out by storage (Redwanz and Walter 1984 ).

However, the use of biogas in a dual-fuel engine, together with the use of diesel, has been investigated. Different parameters are focused on here, for example, the concentration of different exhaust gases (Jaber et al. 2009 ; Owczuk et al. 2019 ), the effect of different fuel mixing ratios (Matuszewska et al. 2016 ). (Lampinen 2004 ) reported on a farm that is completely self-sufficient in energy (power, heat and fuel, and also fertilizers) through the biogas plant. Tractors using biogas as fuel were introduced in the early 2010s. The dual use of biogas and biodiesel to compensate for the low energy density of biogas saves up to 40% in fuel costs, according to one manufacturer (Kruczyński et al. 2013 ).

Pathway 3: upgrading of biogas to a gas of higher value

The third pathway is upgrading biogas to a gas of a higher value. Upgrading of biogas to biomethane is considered a mature technology (Verotti et al. 2016 ). The upgrading process mainly involves the separation of various impurities from the biogas. The most important part of the upgrading process is the separation of CO 2 , with the aim of increasing the methane content of the biogas (Bragança et al. 2020 ). The order of the individual steps for upgrading depends on the gas properties and the method of capturing the CO 2 (Deutsche Energie-Agentur GmbH 2019a ). Upgrading methods are water scrubbing, chemical scrubbing, physical scrubbing, pressure swing adsorption and membrane separation (Miltner et al. 2017 ). Biomethane provides the advantage of multiple applicability, thus, it can be used as a fuel in the mobility sector, as a natural gas substitute in the heating sector, or off-site in a CHP unit. Upgrading leads to higher costs, higher energy consumption and more consumption of material. In addition, methane slip can worsen the environmental footprint (Schmid et al. 2019 ).

The upgrading allows the injection of the gas into the existing gas grid. This provides the advantage that it can be used spatially and temporally independently of its production. This can help to make the use more efficient and, thus, increase sustainability, as a result of demand-orientated energy generation (Budzianowski and Budzianowska 2015 ). Another advantage is that the infrastructure for distributing and using the biomethane has already been established (Cavana and Leone 2019 ). Feeding biomethane into the grid is already a common practice in countries such as Germany, Sweden, the Netherlands, Switzerland and Austria (Korberg et al. 2020 ). The upgrading of biogas to biomethane becomes cheaper with increasing biogas volumes due to cost degression in investments costs; therefore, this pathway is particularly interesting for large biogas plants. There is the possibility of plant pooling, so that smaller biogas plants can also take advantage of the cost degression. Other biogas plants in the vicinity, which can cooperate for joint upgrading, are essential for this (Dotzauer et al. 2021 ). Proximity to the natural gas grid is important for the success of this utilization pathway (Pasini et al. 2019 ). One way to implement pooling is to sell raw biogas to the operator of the upgrading plant. This requires the construction of microgrids to connect the biogas plants to the upgrading plant. In this scenario, the biogas plant operator is therefore only responsible for the production of the biogas, which is subsequently upgraded by another market participant (Dotzauer et al. 2021 ).

In addition to upgrading following the biogas production (ex situ), it is also possible to produce biomethane in the digester, so-called in situ processes. Examples are CO 2 desorption, the pressure reactor, H 2 (hydrogen) addition and electromethanogenesis. For H 2 addition, for example, the H 2 is fed into the digester. The aim is to achieve biomethane of natural gas quality directly with the help of bacteria inside the digester (Aryal et al. 2018 ). Initial studies indicate that in situ methods can offer better profitability for small to medium-sized plants and achieve a methane content of 85%. However, in situ methods are still underdeveloped and currently mostly take place on a laboratory or microscale. Above all, research into the technology on a large scale needs to be improved (Sarker et al. 2018 ).

There is a possibility to further use the captured CO 2 , which is currently not a common practice (Billig et al. 2019 ). The biogenic CO 2 does not cause any climate-relevant emissions and is suitable for various utilization pathways itself (van Basshuysen 2016 ). Possible areas of usage would be the food, chemical and pharmaceutical industries (Horschig et al. 2019 ) or the production of synthetic fuels (Eggemann et al. 2020 ). Capture and subsequent storage of CO 2 , known as Bioenergy with Carbon Capture & Storage (BECCS), is one option for making biogas production CO 2 -negative (Rosa et al. 2021 ).

Under certain circumstances, it may be necessary to adjust the calorific value of the biomethane (e.g., by adding liquid gas). The biomethane must be compressed according to the pressure level of the grid for injection into the natural gas grid. In summary, regarding injection, the quality of the gas must comply with the provisions of the gas class and the deviations must not exceed the permissible limits (Deutsche Energie-Agentur GmbH 2019a ).

Another technical option is to upgrade biogas to green H 2 . Technical options for realization include reforming (steam, partial oxidation and autothermal reforming), biological processes (bio-photolysis, dark fermentation and photofermentation), thermochemical processes (pyrolysis, gasification, combustion and liquefaction) or water splitting (electrolysis, thermolysis and photolysis) (Nikolaidis and Poullikkas 2017 ). The advantage of reformation is that it is a proven technique, as it is conventionally an established process to produce syngas from natural gas (Wünning 2021 ).

Material and methods

A systematic literature analysis of possible business models and economically considered utilization pathways was carried out to obtain an overview of the established business models in the biogas industry. The EBSCOhost database was used for the systematic literature review. The main search term “biogas” had to be included in the title of all candidate papers. Various additional search keywords were used that had to be included in the title or abstract of the paper. The search terms include “business model”/“business case”/“economic”, “use”/“utilization”/“usage” and “pathway”/“path” in various combinations. The search was conducted between September 22 and October 18, 2021.

As a result of the systematic literature review, 242 scientific publications were found; after sorting out duplicate papers, 192 publications remained. The aim of the paper was determined by reviewing the title, abstract and keywords. On this basis, the relevance of the paper was determined for further investigation. Relevant for the selection of a publication was the clear consideration of the profitability of a business model. Publications addressing technical utilization paths or the ecological assessment were not considered further. Regarding the relevant papers, the literature utilized (backward search) and further developments in the research field (forward search) were examined for additional relevant publications. In conclusion, a set of 72 relevant papers remained for the detailed analysis.

Figure  2 shows that interest in biogas business models and utilization pathways is mainly centered in the European region (64% of the relevant papers). Most articles in Europe deal with Italy, Sweden and Germany. A good third of the papers do not deal with any specific country in their methodology.

Distribution of papers by countries and regions considered

An initial classification of the publications into the three main biogas pathways: CHP usage, direct use or upgrading to a higher quality gas was made. If papers deal with several of these topics, they were included in more than one category. The papers in the three categories were examined regarding their publication date and the region under consideration and then classified again. The aim here is an evaluation according to the chronological course and geographical characteristics of individual usage pathways.

Different utilization pathways are covered in the papers, with a focus on CHP usage ( n  = 30). Another utilization pathway that has been studied frequently is biomethane ( n  = 26), with a focus on fuel production ( n  = 16) and CHP off-site usage ( n  = 6). In this context, the use of captured CO 2 was also investigated in seven papers. Five papers investigated the use of biogas to produce H 2 .

Figure  3 shows that the number of relevant papers has increased over the last few years. The research interest of direct utilization of biogas has remained relatively constant over time. The interest in CHP use has increased significantly until 2015, which has shifted over time to the field of biogas upgrading. While almost 40% of the publications address the CHP business model, as well as the upgrading of biogas, the share of direct use is comparatively low at 20%.

Development of relevant published papers by utilization pathway over time

The CHP generation is currently the biogas application pathway most widely discussed. It is treated in many publications in the literature. Thirty of the relevant 72 scientific papers deal with this utilization pathway. The first publication on biogas CHP usages dates back to 2010 (Bystricky et al. 2010 ), and since 2013, there has been an increasing number of publications in this field (Hahn et al. 2014 ; Szarka et al. 2013 ). The number of publications has been relatively constant since 2015. Research interest is concentrated in Europe, especially Germany ( n  = 15, e.g., (Butemann and Schimmelpfeng 2020 ; Theuerl et al. 2019 )), Italy ( n  = 3, including (Gandiglio et al. 2016 ; Patrizio et al. 2017 )) and Austria ( n  = 3, including (Saracevic et al. 2018 ; Stürmer et al. 2021b )).

The aim of flexibilization of CHP units is primarily the power demand-orientated mode of operation, although various utilization pathways or mixed forms are also considered (Fig.  4 ). Hahn et al. ( 2014 ) and Häring et al. ( 2017 ) investigated the various options for flexible power production from biogas with the aim of meeting the requirements and demands of the power market. Grim et al. ( 2015 ) also compared the on-demand production of power to baseload operation and evaluates technical requirements and economic impacts of on-demand production. Dzene and Romagnoli ( 2015 ) showed the possibilities of using biogas to balance the volatile feed-in from wind energy, whereas Bär et al. ( 2020 ) pursued the goal of creating synergy effects from photovoltaics and biogas plants, with the aim of achieving a second-by-second balancing of volatile generation. The first publications on the subject of flexibilization deal with flexible production in Germany (Hahn et al. 2014 , 2015 ; Szarka et al. 2013 ), as incentives have been set to produce power demand-orientated in the EEG (Renewable Energy Act) 2012. However, the research interest in flexibilization in other countries started in 2015, for example, in Latvia (Dzene et al. 2014 ) and Sweden (Grim et al. 2015 ). Research interest in this utilization pathway exists primarily in Germany, as more than half of the publications present the utilization pathway using the German example (Fig.  5 ) (Güsewell et al. 2021 ; Häring et al. 2017 ; Hijazi et al. 2019 ).

Aim of flexibilization of CHP plants investigated in studies, n  = 19

Allocation by country for studies on CHP flexibilization, n  = 19

The direct use of biogas has been studied regularly in the literature since 2004. Seventeen relevant scientific publications were found in the literature search. They can be divided into two categories: the use of biogas in households, for example, for cooking or heating ( n  = 10), or in tractors or agricultural machinery ( n  = 7). The biogas for use in households is normally produced in a decentralized manner and used directly, for example, for cooking (Hamid and Blanchard 2018 ), but also to supply power or produce fertilizer for agricultural purposes (Das et al. 2016 ). Another goal of decentralized plants can be waste disposal, for example, of wastewater (Bensah and Brew-Hammond 2010 ) or to reduce greenhouse gas (GHG) emissions (Yuan et al. 2015 ). Research interest in biogas plants for household use is concentrated in Africa (Kenya (Hamid and Blanchard 2018 ), Ghana (Bensah et al. 2011 ; Bensah and Brew-Hammond 2010 ) and Africa in general (Kemausuor et al. 2018 )), and Asia (Bangladesh (Das et al. 2016 ), China (Chen et al. 2012 ; Yuan et al. 2015 ), Indonesia (Silaen et al. 2020 ), Bali (Bößner et al. 2019 ) and Pakistan (Yasmin and Grundmann 2019 )).

Twenty-six relevant papers were identified for the biogas upgrading pathway. The first publications date back to 2010 (Bystricky et al. 2010 ) and 2012 (Roose et al. 2012 ). An increasing research interest has been observed since 2015 (Budzianowski and Budzianowska 2015 ; Patrizio et al. 2015 ). The geographical focus is on European countries ( n  = 16). There is a clustering for Italy ( n  = 5, e.g., (Cavana and Leone 2019 ; Patrizio and Chinese 2016 )), Germany ( n  = 4, e.g., (Billig et al. 2019 ; Theuerl et al. 2019 )) and Denmark ( n  = 2 (Fenton and Kanda 2017 ; Korberg et al. 2020 )). Only three papers considered a region outside Europe: the USA (Murray et al. 2017 ), India (Bhatia et al. 2020 ) and Thailand (Wattanasilp et al. 2021 ). The research interest in Italy can be justified by the fact that the focus of biogas use has been set on biogas upgrading for use in the transport sector and a support program has been introduced since the Italian National Energy Strategy of 2017 (Murano et al. 2021 ).

There are different focuses on the upgrading of biogas in the literature, like the comparison of different biogas utilization pathways to the upgrading to biomethane (Horschig et al. 2019 ; Kalinichenko and Havrysh 2019 ; Wu et al. 2016 ), the potential role of biogas in the mobility sector of the future (Korberg et al. 2020 ) or the optimal process route, ranging from the upgrading technology to the transportation and utilization method, for biogas (Mohtar et al. 2021 ). The environmental impact can also be relevant to the profitability of the business model, CO 2 prices can lead to higher costs for fossil fuels, so that biomethane becomes more profitable in comparison due to high GHG savings (Patrizio et al. 2017 ). National subsidies that contribute to the profitability of biomethane business models are also explored in the literature (Budzianowski and Budzianowska 2015 ; Patrizio and Chinese 2016 ).

The literature search produced comparatively few hits on the business model of H 2 production. The five papers found are comparatively current from the years 2021 (Cvetković et al. 2021 ; Karaeva 2021 ; Wünning 2021 ) and 2020 (Antonini et al. 2020 ), only one publication dates back to 2013 (Wulf and Kaltschmitt 2013 ). No geographical focus of the research could be identified.

Biogas business models

In the energy industry, especially in oil and gas production, a distinction is made between upstream and downstream business models. The upstream business models involve the identification, extraction or production of raw materials, while the downstream business models follow on from production and primarily involve sales to consumers (Bern 2012 ). This analysis will be limited to the downstream business models. The profitability is considered for agricultural biogas plants, biogas production by wastewater or organic municipal waste is not included in detail.

Possible business models are evaluated starting from the structure given by the three utilization pathways presented in Biogas utilization pathways . Several business models within each pathway were identified in the literature.

The literature review revealed that CHP use can be divided into two basic modes of operation, constant and flexible generation. Following this categorization, the results of the literature review are presented below. Flexible generation can be further categorized according to the goal of the flexibilization. The focus is usually on power-led operation (Bär et al. 2020 ; Lauven et al. 2019 ; Szarka et al. 2013 ), but heat-led operation is also being investigated (Ertem and Acheampong 2018 ; Güsewell et al. 2021 ).

Business model 1.1: constant feed-in with heat concept

It is possible to generate a constant amount of energy in the CHP unit, the so-called baseload operation. Currently, about 75% of biogas plants in the EU use the biogas for CHP production (Calderón et al. 2021 ). The advantage of constant CHP generation over flexibilization is the low investment costs and the significantly lower effort required to operate the plant. However, the low costs do not usually lead to profitability, which would only be possible through subsidies (Lantz 2012 ). In comparison, the revenues in a flexible operation are always higher than in a baseload operation (Grim et al. 2015 ). As the European regulations show, the aim is not baseload operation as this is less beneficial to the energy system. Financial support is given to flexible power generation, with the aim of making the power from biogas usable for the system (Stürmer et al. 2021b ). The long-term viability of this business model is, therefore, questionable.

Business model 1.2: demand-oriented flexibilization

The second way to operate a CHP unit is to generate energy according to a demand or a price signal. Compared to other renewables that generate and feed in power in a volatile manner, biogas offers the possibility of being stored and thus generating energy flexibly and in accordance to demand (Szarka et al. 2013 ). In the case of flexibilization of biogas plants, the literature usually reports on flexibilization with the aim of its usage in the power market. By contrast, there is the possibility of a heat demand-oriented flexibilization of the biogas plant.

Thus, the goal is the balancing of volatile feed-in from other renewable energy sources such as wind turbines and PV plants (Güsewell et al. 2021 ), or the balancing of the residual load and the provision of system services (Szarka et al. 2013 ). In the energy system of the future, flexible power from biogas can reduce overall costs (Fleischer 2018 ) and can help operators generate additional revenue by selling power during periods of higher prices, potentially helping to make the plant profitable (Lauven et al. 2019 ). The revenues can be generated on the markets for ancillary services, e.g., as secondary or tertiary control reserve (Saracevic et al. 2018 ), or on the short-term markets, such as the day-ahead or spot market (Hochloff and Braun 2014 ).

The profitability of the business model is influenced primarily by the amount of the available overcapacities in comparison to the nominal load, schedule design and the amount of external heat use (Daniel-Gromke et al. 2019 ). For example, revenues can be increased by installing a thermal storage system (Wille-Haussmann et al. 2010 ). Furthermore, the amount of additional revenue is influenced by the size of the biogas storage capacity (Lauven et al. 2019 ). The business model of selling power is usually not profitable without subsidies due to declining income from power sales and the high investment costs in storage and CHP (Hochloff and Braun 2014 ; Lauven et al. 2019 ; Lee 2017 ). A positive net present value can be achieved with flexible operation of the existing CHP unit without an investment in new units under certain conditions (Grim et al. 2015 ).

The profitability of a CHP unit increases with high shares of heat utilization, e.g., by nearby industry or households (Goulding and Power 2013 ), so the idea of this utilization pathway is to raise the rate of heat utilization (of the externally available heat) to 100%, if possible (Güsewell et al. 2021 ). In the case of constant energy production by the CHP unit, a high utilization rate can only be achieved with a significantly higher heat demand than covered by the biogas plant (Güsewell et al. 2021 ). In the case of heat demand-oriented flexibilization, the biogas plant is designed and used in such a way that local heat sinks can be covered. Possible heat sinks can, for example, be owned by the biogas plant or neighboring agricultural companies, residential or public buildings, or agricultural drying processes (Herbes et al. 2018 ). The power that is generated simultaneously in the CHP unit is fed into the public power grid and remunerated according to the current tariffs (Daniel-Gromke et al. 2019 ). The actions to adapt an existing biogas plant to this utilization pathway are, firstly, the identification of potential heat sinks in the immediate proximity of the biogas plant, and, secondly, the adjustment of the generation profile to heat consumers. This can be realized by seasonal feeding, for example, adapted by the feedstock amounts used or substrates with different energy density. An alternative method is to use a biogas storage to compensate the fluctuations in the demand curve (Güsewell et al. 2021 ). The disadvantage is that many biogas plants are located in areas where heat utilization concepts are difficult to implement because potential consumers are too far away (Ertem and Acheampong 2018 ).

Short-term flexibilization increases profitability by exploiting higher power prices, while seasonal flexibilization additionally optimizes profitability by increasing heat utilization. However, both options currently do not lead to profitability in Germany under the current market conditions (Güsewell et al. 2021 ).

In addition to optimizing generation to meet an external demand, maximizing on-site energy use can also be a goal of flexibilization. Accordingly, it is necessary to adapt the generation profile of the biogas plant to the on-site load profile (Güsewell et al. 2020 ). The heat generated is currently already used for self-consumption or direct delivery. The idea of this utilization pathway is to consume the energy generated, i.e., power and heat, as completely as possible on-site or in the immediate vicinity. Possible sinks are the farm, residential houses or industrial plants. The aim here is to reduce energy purchase costs so that it is economically more advantageous to consume the energy produced by the biogas plant on-site instead of purchasing power from the grid or thermal energy from other external sources (Güsewell et al. 2020 ). The literature describes that favorable location factors are needed for this model to be profitable. These include low gas production costs and consumers close to the site with a permanently high demand for power. In addition, the possibility of the flexible control of consumer loads and a coupling with another depreciated renewable energy plant contribute to the advantageousness (Technische Hochschule Ingolstadt 2020 ).

The CO 2 price will have a further influence on the profitability of CHP use in the future. Compared to CNG upgrading, CHP use leads to higher GHG savings, which, under favorable conditions, can lead to profitability even with a comparatively low CO 2 price (Patrizio et al. 2015 ).

The direct use of biogas can be divided into two categories: the use of biogas in households, for example, for cooking or heating, or in tractors or agricultural machinery. The use of biogas in households is mainly conducted in Asia or Africa, these family plants mainly pursue the aim to supply households with the needed energy and to replace firewood and dung as energy source (Kemausuor et al. 2018 ). Thus, these biogas plants are especially intended to provide an affordable and reliable source of energy for households (Hamid and Blanchard 2018 ). The use of waste can also be a major goal of a biogas plant (Kemausuor et al. 2018 ).

The biogas utilization in the mobility sector is not common practice without upgrading (Owczuk et al. 2019 ). There is the possibility of utilization in a dual-fuel engine (Jaber et al. 2009 ; Matuszewska et al. 2016 ; Owczuk et al. 2019 ), however, the use of biogas in engines is very limited, making this utilization pathway not sustainable as a business model.

Biogas upgrading usage pathways can be divided into biomethane usage for CHP off-site usage, the heat market, mobility sector and on-site usage. Furthermore, there are the possibilities of CO 2 usage and H 2 generation. The central research interest has been the upgrading of biogas to biomethane, as shown in Fig.  6 . Within biomethane utilization, its use as fuel is currently a focus of research, while the other utilization pathways are under development. The use of captured CO 2 is a research interest that has emerged mainly in recent years.

Share of the utilization pathways considered after biogas upgrading, n  = 44

The investment required for upgrading biogas into biomethane is higher than that for on-site power generation due to the plant technology required for upgrading (Deutsche Energie-Agentur GmbH 2019a ). Upgrading biogas into biomethane is usually only an interesting business model when subsidies are involved since the costs for producing biomethane exceed the market price for natural gas (Budzianowski and Budzianowska 2015 ). However, rising natural gas prices may contribute to the better profitability of biomethane business models in the future (Banja et al. 2019 ). There are already various studies that evaluate the upgrading of biogas to biomethane as the best utilization path and assume that profitability can be achieved (Lee 2017 ).

Business model 3.1: biomethane feed-in for CHP off-site utilization

Basically, power and heat from off-site biomethane CHP units can serve the same markets as on-site CHP units, for example for demand-oriented use or balancing the fluctuating generation of wind and solar power (Budzianowski and Budzianowska 2015 ). The advantage of this option is that the utilization is independent of the location where the biogas is produced, which can lead to higher efficiency due to a higher heat utilization rate, as well as the possibility to use the natural gas grid as biomethane storage (Budzianowski and Budzianowska 2015 ; Horschig et al. 2016 ; Szarka et al. 2013 ). In Germany, off-site CHP use is the most favored utilization path for biomethane with over 85%. The advantage here is the subsidy from the EEG (Renewable Energy Law) for the power generated by the CHP, which can lead to profitability and was so far the best stimulation for biomethane expansion (Daniel-Gromke et al. 2017 ). The range of the CHP units in which the biomethane can subsequently be used extends from 1 kW el to 10 MW el . Thus, the use of biomethane is not only possible in an industrial context; mini CHP units in households are also available on the market (Fachagentur Nachwachsende Rohstoffe e. V. 2012 ). Since the price of biomethane is comparatively much higher than for natural gas and the infrastructure for its use causes the same costs, the substitution of natural gas by biomethane, e.g., in an industrial context, is not advantageous. Thus, profitability in comparison can only be achieved by the customer's decision to choose the more environmentally friendly option. In the future, pricing of CO 2 emissions or alternatively the sale of green gas certificates can contribute to profitability (Patrizio et al. 2015 ).

Business model 3.2: biomethane feed-in for the heat market

In addition to the use of heat from CHP units with a distribution via local heating networks, where the advantageousness depends on factors such as the proximity of the biogas plant to heat consumers and a suitable heat consumption profile, there is the possibility of providing heat via the use of biomethane (Banja et al. 2019 ). In this approach, biogas is upgraded to biomethane in upgrading plants and fed into the natural gas grid. Natural gas plays a central role in the heat supply particularly in countries with well-developed public natural gas grids, especially for supplying households and industry. This existing infrastructure can, thus, be used for the distribution of biomethane, since the latter can be substituted directly for the natural gas that is used throughout the country today (Cavana and Leone 2019 ). Since biomethane is a natural gas substitute, it can be used in conventional natural gas burners. Heating systems or gas stoves, therefore, do not have to be replaced (Banja et al. 2019 ; Fachagentur Nachwachsende Rohstoffe e. V. 2012 ).

In addition to its use in households, the use of biomethane is also interesting in industries that rely on the use of methane-based energy sources for heat generation, such as the iron and steel industry. Biomethane can reduce greenhouse gas emissions in these industries. However, according to current forecasts, an economic substitute is hardly possible even with rising CO 2 prices (Ahlström et al. 2020 ). The profitability of biomethane production depends on plant size, substrate input (Cucchiella and D’Adamo 2016 ) and grid connection costs (Pasini et al. 2019 ), whereby the profitability of different plant constellations could definitely be proven (Cucchiella and D’Adamo 2016 ). The success of biomethane in the heating sector depends primarily on the prices of substitute goods and suffers, above all, from low natural gas prices. However, the trend of rising natural gas prices since 2016 could contribute to better profitability in the future (Banja et al. 2019 ).

Biomethane is currently provided at a higher price than natural gas even under the best circumstances (Paturska et al. 2015 ). In the private sector, therefore, the end consumers' willingness to pay is particularly decisive for the market success of biomethane. For this reason, utilities are currently focusing on gas tariffs with an admixture of biomethane (Dunkelberg 2015 ). Since there is no subsidy for the provision of biomethane in the heat market, e.g., comparable to feed-in tariffs in the power market (Herbes et al. 2021 ), a market ramp-up in this area is hardly to be expected (Adler et al. 2014 ).

CO 2 prices will have a relevant impact on the success of biomethane in the future, especially in direct competition with natural gas. Compared to the direct use of biomethane in the mobility sector, the feed-in into the natural gas grid has the disadvantage that the GHG reduction is reduced by adding propane and thus can be monetized comparatively worse (Patrizio et al. 2015 ).

The issue of profitability is addressed in various studies, which conclude that the use of renewable gases by 2050 will be cheaper than full electrification of the heat sector (Cavana and Leone 2019 ). Other studies, including those which studied the use of biogas in gas boilers in Denmark and the EU, said that heat supply through district heating and individual electric heat pumps are better solutions compared to the use of biomethane. The reason is the more economical cost of the energy system and the biomass use being more economical in other sectors (Korberg et al. 2020 ).

Business model 3.3: biomethane feed-in for the mobility sector

Another utilization pathway for biomethane is its use in the mobility sector. Here, it is possible to feed the upgraded biomethane into the existing natural gas grid and then make it available virtually at natural gas filling stations. This utilization pathway is already common today; mixed products of biomethane and natural gas are usually sold at gas stations (Fachagentur Nachwachsende Rohstoffe e. V. 2012 ). Compared to gasoline, the use of biomethane can save GHG emissions, reducing them by 60% when produced from corn, 70% when produced from waste and about 80% when produced from manure (Banja et al. 2019 ).

The future development of this sales channel depends on the development of CNG vehicles (Fachagentur Nachwachsende Rohstoffe e. V. 2012 ). However, biomethane can be used not only in passenger cars; there are already other vehicles that run on natural gas. These include, for example, light commercial vehicles, trucks, buses (Natural & Bio Gas Vehicle Association 2019 ) and ships (Backman and Rogulska 2016 ). The number of natural gas vehicles in Europe has been steadily increasing in recent years, reaching 1.46 million in 2020, of which 1.25 million are passenger cars. The market share of natural gas cars is the largest in Italy with 2.49% of the total stock of cars. Italy has the largest absolute number with 981,000 CNG-powered cars. Germany has the second largest number of CNG-powered cars in absolute terms (83,000), but they account for only 0.17% of the total car population (European Alternative Fuels Observatory 2020 ). Sweden has by far the largest share of CNG-powered buses with 17.6%, while the Czech Republic (6.7%) and the Netherlands (6.0%) also use CNG-powered buses (European Automobile Manufacturers Association 2021 ). Due to the partial lack of infrastructure in the area of natural gas filling stations and the associated investment costs, as well as the remaining GHG emissions, it is partly assumed that the future of mobility will instead be shaped by electromobility (Banja et al. 2019 ).

It has been shown that various influencing factors determine the profitability of biomethane use in the transport sector. These include primarily subsidies, but also plant size and substrates. Larger plants are generally more likely to be profitable, as is the use of residual and waste materials (Cucchiella et al. 2018 ; D'Adamo et al. 2019 ). Lower taxes for biomethane also offer advantages for profitability (Browne et al. 2011 ). Several studies have shown that biomethane can already be competitive with other fossil fuels or liquid biofuels (Goulding and Power 2013 ). Compared to liquefaction, injection into the natural gas grid is generally cheaper if connection costs are low (Pasini et al. 2019 ).

There is also the possibility of liquefying the biomethane at the points of use to produce bio-LNG (Liquified Natural Gas) (Hönig et al. 2019 ). Bio-LNG can also be used in the transport sector and is particularly interesting for ships or heavy-duty transport due to its higher energy density. Again, a large biogas plant and the use of residual and waste materials as substrates tends to be more profitable (Deutsche Energie-Agentur GmbH 2019b ).

Business model 3.4: decentral biomethane filling stations

As an alternative to feeding the biomethane into the natural gas grid, there is the possibility of setting up a gas filling station near the biogas or upgrading plant. The aim here is also to use the biomethane as a fuel in the mobility sector. This way has been less established so far, but there are already examples in Germany (Fachagentur Nachwachsende Rohstoffe e. V. 2012 ). Customers for the biomethane fuel in the proximity who have their own machines and vehicles, but also logistics companies or bus fleet operators are relevant for the success of this sales channel. In addition, there should be sufficient space for the construction of the upgrading plant and the biomethane filling station, and good accessibility for the customers should be ensured. If there are bottlenecks in the supply of biomethane by the biogas plant, a connection to the natural gas grid should be established (Grösch et al. 2020 ). This can also be used to feed in biomethane during periods of lower sales. Alternatively, it is necessary to ensure an alternative use or storage (Hornbachner et al. 2009 ).

Business model 3.5: on-site usage of biomethane

In addition to the possibility of selling the biomethane produced, it can also be used on-site. The possibilities for use are similar to the on-site use of biogas (Pathway 2) and are primarily intended to contribute to reducing energy procurement costs and increasing the degree of energy self-sufficiency. Thus, biomethane can be used as a fuel in agriculture, replacing diesel and biodiesel. This has a high potential to reduce GHG emissions, contribute to a higher share of renewable energy in the agricultural sector and, thus, to a more sustainable agriculture (Bisaglia et al. 2018 ). Initially, there were tractors that were converted to use biomethane, and dual systems for the joint use of diesel and biomethane were also used (Bisaglia et al. 2018 ). In the meantime, a first series tractor that runs 100% on biomethane has reached market maturity (New Holland Agriculture UK 2021 ). However, the business model is not widespread, so there is no reliable information on profitability.

Business model 3.6: CO 2 utilization

Carbon dioxide is captured during the upgrading of biogas to biomethane. While the captured CO 2 is currently usually released into the atmosphere and not used further, it is also possible to use the captured CO 2 as well. This offers the opportunity in the field of carbon capture and utilization to make a further contribution to climate protection and replace fossil CO 2 generation (Billig et al. 2019 ). There are different ways to use and monetize CO 2 . Large amounts of dry ice are used, especially in the food industry, dry ice service companies and the chemical and pharmaceutical industries (Horschig et al. 2019 ). There are already initial studies showing that it is possible to produce CO 2 for food market utilization. Since the food market offers the most restrictive quality requirements, it would be therefore also conceivable to produce CO 2 for other markets. The CO 2 can also be used, for example, in Fischer–Tropsch synthesis for the production of cosmetic products or for the production of high-value chemicals and waxes (Horschig et al. 2019 ). It is also possible to use CO 2 to produce synthetic fuels. Power-to-fuel, for example, can use carbon to produce renewable fuels together with H 2 via methanol synthesis (Eggemann et al. 2020 ). By selling the CO 2 , the joint use pathway with biomethane can become profitable (Esposito et al. 2019 ). Alternatively, the capture of CO 2 can also lead to negative CO 2 emissions in biogas production. The BECCS process is considered profitable (Li et al. 2017 ). If regulations in the area of the carbon emission trading systems are strengthened, additional revenue can be generated through the negative CO 2 balance (Carranza-Abaid et al. 2021 ; Lisbona et al. 2021 ).

Business model 3.7: hydrogen production

Green H 2 is central in the discussion about energy transition. Comparing the possibilities of producing H 2 from biogas, steam reforming leads to twice the H 2 yield compared to power generation with subsequent electrolysis (Wünning 2021 ). The H 2 is currently transported via trucks as there is no existing H 2 grid. An alternative to this could be the decentralized production of biomethane with feed-in to the natural gas grid and subsequent decentralized upgrading to H 2 . On the other hand, decentralized production can also be an advantage for the nationwide production and distribution of H 2 for use in the transport sector (Wünning 2021 ). In the long term, it is being discussed whether the natural gas infrastructure can be used to distribute H 2 leading towards the elimination of the use of fossil fuels (Dodds and Demoullin 2013 ).

Due to the existing technology of the steam reforming, which already produces hydrogen by using natural gas, it is a market-ready and profitable technology. Compared to using natural gas, biomethane is more expensive but can also result in negative GHG emissions, which in turn can contribute to profitability (Braga et al. 2013 ). The payback period is less than 10 years (Braga et al. 2013 ; Montenegro Camacho et al. 2017 ) and the net present value is positive (Yao et al. 2017 ), making it possible to operate profitably over its lifetime. With production costs below 5 €/kg, biogenic hydrogen is considered competitive (Marcoberardino et al. 2018 ; Montenegro Camacho et al. 2017 ).

The sales markets for hydrogen are diverse and will continue to grow in the future. In industry, hydrogen is used, e.g., in refineries, in chemical production or in the future also in the steel and iron industry, as these will renounce the use of natural gas and coal in order to decarbonize. Furthermore, hydrogen is suitable for generating high-temperature heat (Noussan et al. 2021 ). In the transport sector, there are already passenger cars that run on hydrogen (Alves et al. 2013 ; Antonini et al. 2020 ). In the future, however, the need is seen more in applications that are difficult to electrify, such as trucks, buses, ships and aircraft. Hydrogen is also expected to play a relevant role in the energy system for flexible power generation in the future (Noussan et al. 2021 ).

There has been research interest in utilization pathways and business models in the field of biogas plants since 2004. However, this interest has increased steadily only since 2009. Considering that biogas plants were already being built in European countries in the 1980s (Demuynck and Nyns 1984 ), it is initially surprising that the research interest has only increased strongly in the last decade. However, during the period of this development, new requirements of the energy system have also emerged.

The systematic literature research has shown three basic ways of utilization: the utilization of biogas in a CHP plant, the direct utilization of biogas and the upgrading to a gas of a higher quality. These three utilization pathways contribute in different sectors. Flexible generation of biogas ( Business model 1.2: Demand-oriented flexibilization ), for example, could provide a system service in the power market by generating power from biogas during hours of low power generation from wind and photovoltaics. Many papers have already analyzed this business model extensively. However, compared to other renewables, biogas has high power generation costs and different studies conclude that it is unlikely to be competitive in the long term. On the other hand, it has also been shown that flexible power generation can be profitable under beneficial conditions, such as a high thermal and biogas storage. Another important influence are rising power prices due to rising costs of fossil fuels and rising CO 2 prices due to high GHG savings in CHP usage. Furthermore, the development of technologies to provide system services in the power market will have a relevant influence on the biogas usage in the future. A distinction has to be made between technologies for the short-term and long-term provision of flexible energy. It can be assumed that power storage and flexible generation from H 2 and biogas will complement each other.

Another option that has been considered in the literature is the direct use of biogas ( Pathway 2: Direct usage of biogas ). The option of using biogas in tractors and machinery does not seem very promising, as the lower energy density of biogas does not lend itself well to mobile applications. The use in households for heating, cooking and lighting is limited to low-income countries in Asia and Africa, where the energy supply is not sufficiently developed, especially in rural areas. In Europe, however, the focus is on CHP generation. Therefore, it is not expected that direct use of biogas will be a relevant business model in the future.

The last option is the upgrading of biogas ( Pathway 3: Upgrading of biogas to a gas of higher value ). The use of biomethane offers the advantage of versatility and distribution via an existing infrastructure. Many papers consider possibilities and potentials of the use of biomethane and compare it to alternative use pathways for biogas. A special focus is on the use in the mobility sector. The use of biomethane is of particular interest for vehicles that cannot be powered by power, such as ships or aircraft. The profitability of the use of biomethane in the various pathways currently depends on subsidies or the consumer decision for a green product. The future development in the area of GHG neutrality is relevant here. In a GHG-neutral energy system, biomethane can be relevant to cover methane-based needs in an industrial context. However, a carbon–neutral energy system of the future cannot work with natural gas, as it generates GHG emissions when used. Therefore, the long-term perspective of using the natural gas grid for biomethane is uncertain. In the long term, it could be advantageous for biogas plants to cover regional methane demands with biomethane, for example in the mobility sector in the form of filling stations or industrial demands.

Another advantage of upgrading biogas to biomethane is that the captured CO 2 can be made usable, thus, generate additional revenue. This CO 2 can be used as a substitute for fossil-generated CO 2 . A contribution in the area of carbon capture and storage would also be conceivable here, so, CO 2 could be specifically absorbed in cultivated plants, which would then be used in biogas plants. The CO 2 separated in the upgrading process could then be stored and, thus, removed from the environment. Consequently, biogas plants could contribute to energy production and the reduction of CO 2 in the atmosphere. The separation and utilization of CO 2 can contribute to the profitability of biomethane use in the future. This utilization pathway is influenced by the acceptance of use of biogenic CO 2 , e.g., in the food industry, and the costs and revenues of CO 2 storage.

The topic of generating H 2 from biogas ( Business model 3.7: hydrogen production ) is still of little consideration in the literature from the point of view of business models. However, since the technology for upgrading is already available, this utilization pathway would be feasible in the short term. Biogas could find a long-term sales market in a hydrogen based energy system. First studies show that biogenic H 2 can be profitable and competitive. Here, however, the competitiveness has to be investigated in detail.

A major unknown component for the future development of business models for biogas plants is national legislation. This can lead to a certain utilization pathway becoming financially more attractive. In the EU, for example, the Renewable Energy Directive 2018/2001 (RED II) was issued in 2018, which aims to achieve the climate protection targets—a GHG reduction in transport of 40 to 42% by 2030. Therefore, there should be a share of at least 14% renewable energy in fuel consumption by 2030, as well as a sub-quota of 3.5% for advanced biofuels from residues such as straw and manure (European Parliament and Council of the European Union 2018 ). Biomethane from residues can play a particularly important role in meeting the sub-quota. In addition, financial support for biogas plants in the form of feed-in tariffs for power or biomethane or investment subsidies is also conceivable. This financial support can be differentiated so that, for example, special substrates or technical processes are supported. Tax benefits are also conceivable, giving a competitive advantage to renewables over fossil alternatives, as it is already common practice in Sweden in the mobility and heating sectors. These tax benefits may be tied to minimum GHG reduction standards, for example.

A major influence in the future will be the CO 2 price, which will contribute to the profitability of various potential biogas utilization paths in the long term. On the one hand, the CO 2 price can ensure that fossil alternatives become financially less attractive compared to biogas and, on the other hand, additional revenues can be achieved in the production and sale of biogas through the sale of certificates. Accordingly, the further development of legislation is a major factor influencing the development of the biogas industry.

The use of biogas and associated business models are discussed worldwide, but with a clear dominance of the European perspective. Outside of Europe, the focus is on basic potential analyses of the use of biogas, and in Asia and Africa, also on the use of biogas in households. This business model is not discussed for Europe. The majority of the papers was published within the last decade. During this time, the number of publications has increased steadily, but not explosively.

Furthermore, a development in the focus of the discussion of business models has emerged. The consideration of the direct use of biogas has been relatively constant over the years at a comparatively low level. In Europe, CHP generation is currently the dominant utilization pathway, with an increasing focus on flexibilization since 2015. The dominant aim of flexibilization was to satisfy a power demand. Research on business models for biogas utilization has evolved since 2019, and the focus is now particularly on the upgrading of biogas. Regarding upgrading, many new utilization pathways and business models for biogas have arisen in Europe, after a long period of mainly on-site CHP use. Biogas is becoming more valuable and even more diverse in its possible uses through upgrading. Upgrading biogas to H 2 still represents a small part of the relevant research literature. It is not surprising that research into the field of business models for upgrading biogas to H 2 has been increasing in recent years especially with the increasing demand for H 2 expected in the future.

The systematic literature research has shown that no business model proves profitable under all circumstances, so attention must generally be paid to the individual case of the biogas plant. Direct use is only a partially interesting business model in regions without a well-developed gas distribution infrastructure. The possibilities of flexible CHP use and upgrading appear promising in the literature. Future use of CHP is only viable with high flexibility. If fossil options for flexible generation are abandoned in the future, the synergies of different flexibility options can be used to provide system services. Thus, biogas plants can play a crucial role, especially in the power sector of the future. On the other hand, upgrading biogas to biomethane offers the advantage of multiple uses. Especially in applications where there are no other green alternatives, biomethane could play a decisive role in the future. For example, biomethane can meet methane-based needs in industry or serve as a fuel for heavy-duty transportation. In the future, however, biomethane upgrading should always be accompanied by CO 2 capture and use or storage, with the goal of increasing profitability and environmental benefits. Upgrading of biogas to hydrogen has not been investigated much yet, but could play a significant role in the future. Further investigations are needed to determine whether upgrading to hydrogen has any advantages. Factors influencing the profitability of a given business model thus include the size of the biogas plant, since upgrading biogas becomes more cost-effective especially as volumes increase, and the availability of local customers for heat or generated biomethane.

The development of the CO 2 price will be relevant for biogas plants in the future. On the one hand, this has an influence on the amount of revenues by trading certificates for the reduction of GHG emissions and, on the other hand, on the competition with fossil fuels, which become comparatively more expensive due to a higher CO 2 price, resulting in a competitive advantage for biogas. However, as various studies have shown, political support instruments are particularly relevant for a development of a country's biogas plant stock. Thus, the promotion of a certain business model leads to an increasing implementation of this utilization pathway. In the interest of the national GHG reduction strategies, it would be useful to set clear signals for the future development of existing plants, so that plant operators can develop a clear long-term strategy for the use of their plants.

The utilization pathways for biogas have become more diverse and each of these pathways has advantages. However, there is competition between the individual pathways since biogas as an energy resource is finite. The future political framework is a particularly relevant influencing factor on the development of the biogas industry. It has already led to the biogas industry developing differently in different countries as a result of various support programs. It needs a consistent political framework for a target-oriented, further development of the industry in the next few decades. The system utility of the use pathways can be a major argument for use in a particular energy sector. The environmental impact of biogas use will also have a greater influence on utilization decisions in the future; the goal of integrating it into a circular economy is already being explored.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Mertins, A., Wawer, T. How to use biogas?: A systematic review of biogas utilization pathways and business models. Bioresour. Bioprocess. 9 , 59 (2022). https://doi.org/10.1186/s40643-022-00545-z

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A critical review of biogas production and usage with legislations framework across the globe

S. abanades.

1 Processes, Materials, and Solar Energy Laboratory, PROMES-CNRS, 7 Rue du Four Solaire, 66120 Font-Romeu, France

H. Abbaspour

2 Department of Biology, Faculty of Biological Science, North Tehran Branch, Islamic Azad University, Tehran, Iran

3 School of New Technologies, Iran University of Science & Technology, Tehran, Islamic Republic of Iran

4 Department of Mechanical Engineering, National Institute of Technology Silchar, Silchar, Asaam 788010 India

M. A. Ehyaei

5 Department of Mechanical Engineering, Pardis Branch, Islamic Azad University, Pardis New City, Iran

F. Esmaeilion

6 Department of Energy Systems Engineering, School of Advance Technologies, Iran University of Science & Technology (IUST), Tehran, Iran

M. El Haj Assad

7 Sustainable & Renewable Energy Engineering Department, University of Sharjah, Sharjah, United Arab Emirates

T. Hajilounezhad

8 Department of Mechanical & Aerospace Engineering, University of Missouri, Columbia, MO USA

D. H. Jamali

9 School of Environment, College of Engineering, University of Tehran, Tehran, Iran

10 R, L. Applied Thermodynamic, National Engineering School of Gabes, University of Gabes, Gabes, Tunisia

H. A. Ozgoli

11 Department of Mechanical Engineering, Iranian Research Organization for Science and Technology (IROST), Sh. Ehsani Rad St., Enqelab StParsa SqAhmadabad Mostoufi RdAzadegan Highway, 3313193685 Tehran, Iran

12 Department of Energy Engineering, Faculty of Natural Resources and Environment, Science and Research Branch, Islamic Azad University (IAU), Daneshgah Blvd, Simon Bolivar Blvd, 1477893855 Tehran, Iran

13 Department of Mechanical and Nuclear Engineering, University of Sharjah, Sharjah, UAE

E. H. Bani-Hani

14 Department of Mechanical Engineering, School of Engineering, Australian College of Kuwait, Kuwait City, Kuwait

This review showcases a comprehensive analysis of studies that highlight the different conversion procedures attempted across the globe. The resources of biogas production along with treatment methods are presented. The effect of different governing parameters like feedstock types, pretreatment approaches, process development, and yield to enhance the biogas productivity is highlighted. Biogas applications, for example, in heating, electricity production, and transportation with their global share based on national and international statistics are emphasized. Reviewing the world research progress in the past 10 years shows an increase of ~ 90% in biogas industry (120 GW in 2019 compared to 65 GW in 2010). Europe (e.g., in 2017) contributed to over 70% of the world biogas generation representing 64 TWh. Finally, different regulations that manage the biogas market are presented. Management of biogas market includes the processes of exploration, production, treatment, and environmental impact assessment, till the marketing and safe disposal of wastes associated with biogas handling. A brief overview of some safety rules and proposed policy based on the world regulations is provided. The effect of these regulations and policies on marketing and promoting biogas is highlighted for different countries. The results from such studies show that Europe has the highest promotion rate, while nowadays in China and India the consumption rate is maximum as a result of applying up-to-date policies and procedures.

Introduction

From the 1980s onward, the striking jump in global energy consumption has been largely driven through fossil energy resources. Generally, oil, coal, natural gas, electricity, nuclear energy, and renewable energies have shared 33, 27, 24, 7, 4, and 4% of total primary energy proportion in the whole world in 2018, respectively. Approximately, 85% of the world's primary energy consumption has been supplied by fossil fuels in 2018 (BP. 2019 ; Ghasemian et al. 2020 ).

The conversion of biomass to energy has been promoting from 65 GW in 2010 to 120 GW in 2019 due to climate change, reasonable energy prices, distributed generation increase, and environmental aspects, in recent years. Wastes with high moisture content are more compatible with conversion by anaerobic digestion, landfill, and digestion technologies. The global amount of biogas plant capacity was about 19.5 GW at the end of 2019. Organic wastes are the most common feedstocks to produce biogas from wastes, including domestic wastes (food, fruits, and vegetables) or public moist wastes (cafes and restaurants, daily markets, and companies’ biological wastes), due to significant moisture content and high degradability. These input materials are classified as OFMSW, which represents the organic fraction of municipal solid waste (Antoine Beylot et al. 2018 ; A. Luca C.R. 2015 ).

Biogas is inherently renewable, on the contrary to fossil fuels, because it is generated from biomass, and this source is practically a reserve of the solar energy via photosynthesis process. Anaerobic digestion (AD) biogas will not only enhance a country's energy basket status but also contribute significantly in conserving natural resources and protecting the environment (Teodorita Al Seadi DR 2008 ).

Biogas is naturally composed of biogenic material. This biogas, which occurs naturally, spreads into the ambient, and its major component, methane, plays a serious detrimental role in global warming (Bochmann and Montgomery 2013 ). Methane has been used as important fossil fuel and converted to generate power, transportation, and heating, over the past decades. Nowadays, the major portion of methane consumption and utilization comes from natural gas resources, but the production of bio-methane from waste recovery approaches has been meaningfully increased. Its production potential has been improved by 4% over 9 years (from 2010 to 2018). At present, about 3.5 Mtoe of biomethane is produced around the world and the potential for biomethane production today is over 700 Mtoe (Edenhofer et al. 2011 ). Of course, this does not mean that methane conversion is feasible from all kinds of natural resources. In other words, infrastructures for biogas development extremely rely on specific equipment and the availability of control and management systems. Therefore, a sustainable industry can be installed and implemented to generate bio-energy from renewable and green natural resources (Bochmann and Montgomery 2013 ).

Developed countries use advanced large-scale plants for utilizing biogas. Biogas is regularly applied to generate heat, power, and electricity. Also, several industrial applications for its utilization in biogas plants as a substitute to natural gas are being progressed. Based on the analyzed data, a continuous increase in biogas production has been observed due to the global policies and programs. Since 0.5% proportion of renewable energies contribution that is about 12.8 GW (IRENA RES. 2015 ) is supposed to be achieved in 2020 for transportation sectors, bio-fuel production has been considered as the main source of this plan in different regions. It is noteworthy that biogas production should not be developed as a food production threat. For this reason, biofuels are mainly generated from cellulosic and lignin wastes (Nicolae Scarlat and Fahl 2018 ; Angelidaki et al. 2018 ).

A wide global market of biogas has been conspicuously promoted for the previous decades in various countries. Moreover, the advanced biogas production technologies have been supported by domestic or international supportive rules, such as research, design, and development (RD&D) financial funds, subsidization, and guaranteed electricity purchase contracts to make a competitive market against conventional energy suppliers (Teodorita Al Seadi DR 2008 ).

According to Fig.  1 , the different utilizations of the biogas technology offer a multi-purpose solution to generate the required energy of the industrial or social sectors. Biogas is mainly consumed for combined heat and power (CHP) plants, hydrogen production units, and advanced energy systems such as fuel cells.

Overview of biogas utilization

Generally, in the European Union (EU) and North America (NA), biogas plants came to be developed more than in other continents for the last 40 years. The main advantages of the units located in the mentioned regions are industrial scale, energy efficiency, and high complexity level. Biogas production was considered by academic centers and governments owing to its potential in response to different global challenges. It should also be pointed out that using biogas technologies allows industries to eliminate greenhouse gases (GHGs) emissions and waste disposal pollutions, while it provides a broad spectrum of energy utilization such as heat, electricity, and transportation purposes, based on its renewable nature.

There are various strategies around the world for producing biogas from agricultural products. In Germany, for example, the production of cheap agricultural products that require low processing (with no outcomes for consumers) provides feedstock for biogas plants. New policies recommend the use of crops and plant residents, life stocks remaining, and landfill use (IRENA RES. 2015 ).

This review focuses on proposing a comprehensive analysis of the recent biogas technologies progress, aiming advances toward wastes conversion to produce electricity, heat, and other forms of energy carriers. It reports the current and future AD conversion technologies, as well as examines accessible details in the literature about feedstock categories, pretreatment approaches, process development, and its yield to increase production efficiency. Furthermore, suggested future biogas application trends and directions for efficient ways of energy generation from wastes are other main outputs of this study. Also, the present review highlights the emerging biogas technologies which are promoted to distribute biomethane and biofuel production, especially the production of hydrogen from biogas is the innovative insight in the mentioned field.

The structure of the present research is as follows: “Biogas Applications” reports extensive data on the up-to-date status of biogas consumption in energy generation, energy storage, and transportation. Biogas development levels around the world, regulations, and historical progress are expressed in “Biogas utilization in various parts of the world” section. Also, the characterization of the feedstocks and additives, pretreatment, process types, and related techniques are described in “Recent progress in biogas production” section. The novel technologies are indicated with their advantages and constraints for each section. Eventually, the conclusion and predictive tendencies for future research are explained in the last section.

Thus, this work represents a comprehensive review of the biogas in terms of a renewable energy source for both production and applications. The procedures for production and applications are up to date. Researchers' work in 2020 is presented where they used the most updated technologies which help other research agencies to continue from this end. The review of the development of the biogas industry and utilization covers 20 years of information. Moreover, a review of the international recent policies and regulations relevant to biogas management is provided. Based on that, a suggested policy based on international guidelines and international conventions is proposed.

Methodology

Published research papers and data on biogas sources, production, and applications are collected from the literature. These sources cover the years from 1997 till 2020 to summarize the current situation and development relevant to biogas. A review of policies and regulations on national and international levels is presented. Regulatory entities in the world that issue guidelines instruction to organize the biogas market are presented. This review showed the increase of world awareness regarding this source of energy by introducing the most updated policies in many countries. Based on all of the above, a proposed framework and policy is presented.

An introduction shows the necessity of biogas as a source of renewable energy is presented. The increasing demand for biogas in the energy section showed to be increased in the coming years. Biogas production process and the sources to get the biogas are presented. The sources vary from agricultural to animal wastes which are the richest biogas sources however, other sources such as wastewater treatment plants, and landfill disposal sites.

Applications of biogas and its contribution to the total national energy sector are presented. These applications range from energy conversion, producing alternative fuels, electricity generation, etc. Traditional methods of biogas production are presented with developments of such methods. New technologies and methods for production and purification of biogas are described.

Biogas applications

Biogas is globally considered as traditional off-grid energy. Biogas can also be utilized to generate electricity. The various applications of biogas are described below.

Electricity generation

Power generation from biomass is currently the most popular and growing market worldwide, due to technological improvements, decreasing reliance on fossil-based energy, and reduction of greenhouse gases (GHG) emissions. Biogas has the potential for electricity generation in power plants by internal combustion engines (ICEs) or gas turbines (GTs) as the two most commonly used power generation methods. Micro gas turbines are also an attractive method due to lower NOx emissions and flexibility to meet various load requirements. Multiple microturbines sizing from 70 kW to over 250 kW can be employed to meet low/medium power load demands. The electricity can provide the required power to the adjacent industries and companies. With the development of electric cars, another state-of-the-art application, especially in developed countries like Germany, is the utility of electricity for e-vehicles of a connected car-sharing association (Scarlat et al. 2018 ).

The major benefit of on-site electricity generation is to prevent transport losses and to increase reliability due to the independence from a centralized grid mostly run by traditional fossil fuels. It also brings extra economical profit by providing the required in-house power demand and selling the extra electricity (Scarlat et al. 2018 ).

Heat generation

Biogas can be directly combusted in boilers for heat generation only. It is feasible to slightly modify natural gas boilers to operate with biogas. As farm biomass is a major biogas production source, the generated heat can be used for heating the digesters, farm buildings like housing units for pigs/sties, greenhouses, as well as aquafarming, cooling/refrigeration of farm products, and drying purposes. The drying process in agricultural businesses, such as drying of digestate, woodchip, grain, herbs, and spices, is a remarkable added value to the farm economy (Herbes et al. 2018 ).

Available heat for external use, representing nearly 30–50% of generated heat, can be sold to a nearby district to be used for district heating/cooling like heating swimming pools. Also, an absorption chiller can be a potential candidate to better use heat through CHP, in addition to cooling power (tri-generation). It can convert heat into cooling power with high efficiencies of up to 70% (Rümmeli et al. 2010 ).

Combined heat and power (CHP) generation

Concurrent generation of heat and electricity by CHP systems is an operational approach to upgrade the energy conversion efficiency of biogas. When only converting biogas to electricity or heat, just a minor fraction of energy contained in biogas is used. Characteristically, in these types of systems, associated power conversion productivity is somewhere in the region of 30 to 40%, while it is diminished by employing biogas as an alternative for refined and purely natural gas (Saadabadi et al. 2019 ).

CHP plants offer the advantage of high-temperature exhaust gas from the electricity generation subsystem (ICEs or GTs) as a source of valuable heat for many heating purposes already discussed before. Although the electricity generation efficiency of simple plants is only 20–45% (Muche et al. 2016 ), a larger portion of energy (around 60% of the utilized energy (Damyanova and Beschkov 2020 )) is converted to heat that is reused by heat recovery systems; making it more attractive when there is a high heat demand. This considerably enhances the system efficiency and improves the payback period of plants, making the distributed generation the most common biogas application. The extra electricity could be supplied for the national grid and the extra heat can be sold to the local district utilization.

A CHP cycle has sufficient productivity that has an efficiency up to 90%, while it can produce 35% and 65% of the generated electricity and heat, respectively. In this case, some thermal energy is used to heat the process and about 2/3 is used for external uses. In some proposed models for biogas-based power plants, the use of generated heat is ignored and the focus is only on generating electricity. Without any doubt, this approach has no economic justification and must use all its thermal potential.

There are three common ways to produce heat and power from biogas including Gas-Otto engines, Pilot-injection gas motor, and Sterling motors (Teodorita Al Seadi DR 2008 ). In EU, four-stroke engines and ignition oil diesel engines contributed roughly the same in CHPs at somewhere in the vicinity of 50%, each (Dieter Deublein 2008 ). Biogas is also employed in gas turbines, microturbines, and fuel cells (discussed in detail in `` Fuel cells '' section ) for CHP applications (Kaparaju and Rintala 2013 ; Nikpey Somehsaraei et al. 2014 ).

CHP plants offer the advantage of high-temperature exhaust gas from the electricity generation subsystem (ICEs or GTs) as a source of valuable heat for many heating purposes already discussed. Although the electricity generation efficiency of simple plants is only 20–45% (Muche et al. 2016 ), a larger portion of energy (around 60% of the utilized energy (Damyanova and Beschkov 2020 )) is converted to heat that is reused by heat recovery systems; making it more attractive when there is a high heat demand. This considerably enhances the system efficiency and improves the payback period of plants, making the distributed generation the most common biogas application. The extra electricity could be supplied for the national grid, and the extra heat can be sold to the local district utilization. Also, an absorption chiller can be a potential candidate to better use the extra heat through CHP, in addition to cooling power (tri-generation). It can convert heat into cooling with high efficiencies of up to 70% (Rümmeli et al. 2010 ).

A CHP cycle has sufficient productivity that has an efficiency up to 90%, while it can produce 35% and 65% of the generated electricity and heat, respectively (Shipley et al. 2009 ). In some proposed models for biogas-based power plants, the use of generated heat is ignored and the focus is only on generating electricity. Without any doubt, this approach has no economic justification and must use all its thermal potential.

Upgrading to biomethane

If biogas is upgraded and purified to biomethane, it can be fed into natural gas grid to be used for heating purposes, power generation, or to provide fuel for compressed natural gas (CNG) and even natural gas vehicles (NGV). A significant benefit of biomethane is that it can be stored to meet peak demands (Herbes et al. 2018 ). The two major steps to produce biomethane are upgrading methane content up to 95–97% followed by a cleaning process to eliminate water vapor, hydrogen sulfide, oxygen, ammonia, siloxanes, carbon dioxide, carbon monoxide, hydrocarbons, and nitrogen (Ryckebosch et al. 2011 ). Biogas upgrading is performed by physical and chemical technologies such as adsorption, absorption, cryogenic and membrane separations, and gas separation membranes as well as biological technologies (in situ and ex situ (Kapoor et al. 2019 )). Although biological methods are emerging, suggesting an enormous technological potential, they are not widely used in industry since they are generally much slower, have low rates of reaction/synthesis, and require long startup period that made them less economically feasible, while physicochemical methods are common due to technological advancements and implementations (Scarlat et al. 2018 ).

Upgrading biogas to biomethane or renewable natural gas (RNG) is on a hot trend in developed countries especially in North America among oil and gas companies for decreasing GHG emissions and using the carbon credit. There are also other environmental and economical benefits in smaller scale to farmers, municipalities, and counties for waste management and profitable contracts with gas utility companies. Biomethane market for transportation purposes equaled to 160 cubic meter per year in 2015 Eurostat.European Statistics ( 2019 ).

Transportation fuel

Biogas converted to biomethane (through upgrading and cleaning) can be readily used in natural gas-powered vehicles as another option for fossil natural gas. Using biomethane as transportation fuel results in remarkably low GHG emissions that make it a suitable source of renewable fuel. Biomethane turns out to be a great fit to replace fossil-based fuels in terms of environmental and economic considerations (Scarlat et al. 2018 ). However, the overall efficiency is extremely improved when biomethane is utilized in advanced hybrid or fuel cell vehicles (FCVs) in comparison to current biodiesel or ethanol-powered ICE vehicles (Faaij 2006 ).

Generally, biogas can be improved to transportation fuels (bio-CNG) that can be stored for future use, in the form of liquefied biogas (LBG), syngas/hydrogen, methanol for gasoline production, ethanol, and higher alcohols (Yang et al. 2014 ). Compression and liquefaction are common physical methods to convert biogas into bio-CNG and LBG, while the dominant chemical approach to obtain syngas is catalytic reforming. If Fischer–Tropsch synthesis (FTS) or fermentation is employed, syngas may be converted into a variety of alcohols like methanol, ethanol, and butanol (Yang et al. 2014 ). This fuel alternative has already been applied within the European Union and the USA. As an example, many vehicles run on biogas in the urban public transport (in Sweden and Germany) either as 100% methane (CBG100) or mixed with natural gas (e.g., CBG10 and CBG50) (Damyanova and Beschkov 2020 ; Yang et al. 2014 ).

Hydrogen production

Hydrogen displays many promising potentials for renewable energy and the chemical industry due to its high potential for energy production. Hydrogen offers the biggest share of energy per unit mass (121.000 kJ/kg). The hydrogen council suggests about 18% contribution of total final energy utilization by 2050. Hydrogen is best employed in fuel cells as an emerging energy application to produce electricity, heat, and possibly water. Furthermore, there are many applications in chemical industries for hydrogen, including food treatment, hydrogenation methods, production of ammonia and methanol, Fischer–Tropsch synthesis, pharmaceutical manufacturing, among others (Armor 1999 ).

Technically, hydrogen (H 2 ) can be released from the BSR (biogas steam reforming) process. This process has temperature flexibility in the range of 600 to 1000° C, which also includes catalytic techniques. (Holladay and J., King, D.L., Wang, Y. 2009 ; Alves and C.B., Niklevicz, R.R., Frigo, E.P., Frigo, M.S., Coimbra-Araújo, C.H. 2013 ). The main difference between BSR and SMR (steam methane reforming) is the presence of carbon dioxide in the feedstock. This factor increases the sensitivity to carbon production in the process. The produced carbon can deposit in the active phase of the catalyst to create deactivation.(Gioele Di Marcoberardino et al. 2018 ). Furthermore, fed gas can affect the hydrogen separation unit. In this case, PSA (pressure swing absorption) and VPSA (vacuum PSA) are the most common methods of purifying the system for hydrogen-rich reformate or syngas (Ugarte and P., Lasobras, J., Soler, J., Menéndez, M., Herguido, J. 2017 ; Ahn and Y.W., Lee, D.G., Kim, K.H., Oh, M., Lee, C.H. 2012 ). The potential of hydrogen production from all landfill sources in the USA is probably between the total potential of 16 million tons of methane from raw biogas and 4.2 million tons of hydrogen (Milbrandt GSaA. 2010 ). Biogas production systems have a capability for production from 100 Nm 3 /h for small-scaled agricultural to a few 1000 Nm 3 /h for large-scaled municipal waste landfills; furthermore, occasionally, not all biogas may be converted to the desired hydrogen and further biogas valorization can coexist in the system. Therefore, the capacity considered for BSR should be in the range of 50 and 1000 Nm 3 H 2 /h (Doan Pham Minh et al. 2018 ).

Hydrogen is clean transportation fuel, while as discussed earlier syngas may be used as a feedstock for alcohol production. With new advancements in reforming procedures, biogas can now be directly improved to syngas by dry or steam reforming without the necessity to remove carbon dioxide (Yang et al. 2014 ).

Fuel cells are probably the cutting-edge application of biogas. Recent advances in fuel cells resulting in low emissions (CO 2 , NO x ) and high efficiency make them suitable for power generation and transportation purposes. Also, fuel cells can be utilized in large-scale power plants, power distribution generators, buildings, small-scaled and portable power supply apparatus for microelectronic equipment, and secondary power components in vehicles (Alves et al. 2013 ).

Fuel cells can use the chemical energy of hydrogen and oxygen without any intermediaries to deliver electricity and heat (A. Trendewicz R.B. 2013 ). In this case, there are only a small number of fuel cell-based power plants (most of which are pilots) that generate electrical power from biogas. (S. Ali Saadabadi ATT, Liyuan Fan, Ralph E.F. Lindeboom, Henri Spanjers, P.V. Aravind. 2019 ). Fuel cells exhibit high electrical efficiency of 60% (in power generation only mode) and thermal efficiency of up to 40% (in CHP applications) (Pöschl et al. 2010 ), but can easily be integrated with other power generation systems like gas turbines or microgas turbines to further improve their performance. Also, biogas fueled integrated solid oxide fuel cell (SOFC)-CHP offers a modern energy system that can address both heat and power generation demands for decentralized grids with drastically higher electrical efficiencies (Wongchanapai et al. 2013 ; Safari et al. 2020 ; Safari et al. 2020 ). Such high efficiency compared to other common combustion technologies is a result of not being limited by thermodynamic Carnot efficiency. SOFCs are more tolerant to fuel impurity and flexibility; hence offering better integration with biogas systems (Wasajja et al. 2020 ). This highlights their key role in enhancing the highly efficient generation of electricity from biogas, which demonstrates significant environmental and economic merits. However, for the use of biogas as fuel in fuel cells, a cleaning procedure seems essential to eliminate biogas impurities such as H 2 S, siloxanes, and other volatile organic compounds (VOCs) that have harmful impacts on fuel cell operation.

Furthermore, hydrogen produced from biogas can directly feed fuel cells. The reforming practice can be succeeded either internally employing fuel cells or externally by a catalytic pre-reformer. The three chief techniques for methane conversion are steam reforming, partial oxidation (POX), and dry reforming. Besides, mixed approaches like autothermal reforming (ATR) (mixed steam reforming and methane POX) are applicable. In a pilot plant constructed in Barcelona, Spain named “Biocell project”, biogas from a WWTP was employed in two categories of a fuel cell. The first was proton-exchange membrane fuel cell (PEMFC) that entailed exterior gas cleaning and reforming unit. Biogas has also been added into a SOFC after the cleaning process. This pilot plant is intended for 2.8 kWe. Electrical and thermal effectiveness for the SOFC pilot plant was 24.2 and 39.4%, respectively, which are considerably more than those for the PEMFC pilot plant (S. Ali Saadabadi ATT, Liyuan Fan, Ralph E.F. Lindeboom, Henri Spanjers, P.V. Aravind. 2019 ; Arespacochaga and CV, C. Peregrina, C. Mesa, L. Bouchy, J. Cortina 2015 ).

Biogas development in various parts of the world

The worldwide biogas industry has increased more than 90% between 2010 and 2018, while further growth is still expected. The International Renewable Energy Agency (IRENA) reported that the overall potential for the biogas industry in 2018 could provide 88 Tera Watt per hour (TWh) of biogas each year. Installed electricity generated from biogas reached 18.1 GW in 2018, against 8.2 GW in 2009 (Agency 2019 ). Over 20% of electricity produced in the entire biopowered production is generated from biogas, with a share of 4% of heat generation worldwide.

Among different countries throughout the world, Europe plays a pivotal role in biogas electricity generation. In 2017, Europe contributed to over 70% of the world biogas generation representing 64 TWh, followed by North America accounting for 15 TWh (in which the US participation was over 85% in entire North America). Asia produced 4 TWh followed by Eurasia with 1.7 TWh, South America with 953 GWh, and Africa biogas production accounted for 89 GWh (Scarlat et al. 2018 ; Agency 2019 ).

In terms of thermal energy production, biogas is turning to be a more significant source of heat, in which around 4% of the worldwide bioheat in 2015 was generated by biogas. In the EU, biogas produced 127 TJ of heat, which corresponds to almost 50% of entire biogas use in the EU (Scarlat et al. 2018 ). In Demark, the electrical power cost produced by biogas is 0.056 EUR/kWh in a CHP unit or injected into the grid (Seadi and J. 2019 ).

Biogas utilization differs significantly in various countries around the world. This varies from several small-scaled biogas plants providing heat in China and India to large-scale plants generating electricity as well as upgrading into biomethane as fuel, mostly in Sweden (McCabe et al. 2018 ).

Nanyang in China is one of the top biogas cities in the globe due to its location in the center of a rank soil zone. Since corn is abundant, other types of cereals can be employed for producing biogas (Dieter Deublein 2008 ; Lei Zheng 2020 ).

In China, biogas plants are classified as medium scale with the volume of digester equaled to 300 cubic meters and large scale with a capacity of 500 cubic meters, with daily biogas production in the range of 150 to 500 cubic meters per day (Song and C., Yang, G., Feng, Y., Ren, G., Han, X. 2014 ). The governmental support for domestic digester has been stopped since 2015. More backing would make large-scale biogas plants and bionatural gas schemes (Ndrc 2015 ). Chinese biogas industry reported that 41.93 million biogas digesters were built (containing centralized biogas source for houses), for almost 200 million recipients, in which 14.5 billion m 3 biogas is produced per year (China Statistics Press 2018 ).

In India, around 2.5 Mio biogas plants are operating, with a medium digester volume of 3–10 m 3 . Based on the circumstances, the plants produce 3–10 m 3 biogas daily, adequate to deliver a regular farmer family with energy for food preparation, heating, and lighting. Also, more than 1.2 million households employ small-scaled AD and 100,000 family-sized AD units have been installed between 2016 and 2017. Over 35,000 biogas plants have been constructed with governmental investments (MNER 2016 ).

Japan is a pioneer in the use of biogas, with increasingly using AD to produce biogas and manage municipal waste in the last decade. The development is such that only Japan uses thermophilic AD (Abbasi et al. 2012 ).

Up to 2008, over 70 plants have been constructed in Russia, over 30 in Kazakhstan, and a single plant in Ukraine. In Ukraine, bioreactors with 162,000 m 3 volume have been previously installed in sewage treatment units (M. R. Atelge DK, Gopalakrishnan Kumar, Cigdem Eskicioglu, Dinh Duc Nguyen, Soon Woong Chang, A. E. Atabani, Alaa H. Al-Muhtaseb, S. Unalan. 2018 ).

It should be noted that some nations employed biogas as a practical tool for waste management, mostly to decrease the detrimental effects of municipal waste or wastewater. Likewise, a broad range of various technologies are employed from simple digesters to expanded granular sludge blanket (EGSB) digesters (McCabe et al. 2018 ).

Biogas technology and industry

The biogas industry varies significantly in the various parts of the world. Different countries have been advanced in several types of biogas systems mainly premised on different environment as well as energy demand and supply chain. The UK, Australia, and South Korea employed landfill sites to achieve a considerable portion of their produced biogas, while in Switzerland and Sweden, using decomposition of sewage to generate biogas is prevailing. Denmark utilizes mainly manure due to its abundance and availability. In Germany, UK and Sweden most of the biogas generation arises from food waste (McCabe et al. 2018 ; Union 2015 ; Association WB.Global Potential of Biogas 2019 ).

In farm-based biogas production, China and Germany are recognized as world leaders since about 24,000 small-scale plants exist in China and nearly 8000 agriculture plants in Germany. Similarly, France, Holland, Austria, and Italy employed considerable farm-based biogas plants (Union 2015 ). Moreover, the scale of plants ranges from small household units to larger plants using feedstocks such as household waste, industrial waste, and manure to generate both heat and electricity (Union 2015 ). Studies revealed that in Asia and Africa, most of the installed biogas plants were family-sized (Kemausuor et al. 2018 ). China and India have dominated the microscale biogas industry in the world. At this time, Thailand takes benefits from more than 1700 biogas plants and more than 150 plants of industrial waste. The Thai government has attempted to expand industrial wastewater technology that has the potential of 7800 TJ/y biogas production (Tonrangklang et al. 2017 ). The ministry of energy of Nepal (Government of Nepal Ministry of Energy WRaI.Biogas. 2020 , 2020 ) has reported that most of the villages about 2800, out of the total 3915 in all 75 districts of Nepal, have small-scale or household biogas production systems. Primarily two categories of plants have been constructed in Nepal. These are the floating-drum plant based on the Indian style and fixed-dome plants with a flat floor, cylindrical digester, and a dome prepared by concrete. Among 50 million microscale digesters operating in various parts of the world, 42 million are installed in China and another 4.9 million in India. The statistics from the World Biogas Association (WBA) have shown that there are only 700,000 biogas plants installed in Asia, Africa, and South America (Association WB.Global Potential of Biogas.2019. 2019 ).

In terms of large-scale plants, about 7000 large-scale biogas systems are operating in China. Europe, in 2017, had a share of 17,783 plants, while Germany was dominating the European biogas industry with 10,971 plants followed by Italy with 1665 plants, France with 742, Switzerland, and the UK with 632 and 613 plants, respectively (Association 2018 ). The World Biogas Association data mentioned about 2200 anaerobic digesters large-scale plants in the USA, able to generate 977 MW (Association WB. International Market Report 2018 ).

Another application of biogas relies on upgrading to biomethane. Although being comparatively a novel technique, it achieves widespread utilization worldwide. Some biogas upgrade plants are employed to produce vehicle fuel, while others deliver it into the local or national grids Association WB.Global Potential of Biogas ( 2019 ).

Africa is a region with abundant and diverse resources for biogas production, though it has accomplished small progress in the sector. Although the continent has made considerable achievements in small-scale biogas plants, profitable biodigesters still require further development (Kemausuor et al. 2018 ). In Africa, harvest and livestock farmers, small to medium and large food treating businesses, wastewater, sanitation, and municipalities running institutes, as well as municipal waste management organizations, are considered as potential candidate employers of large-scale biogas technology. Moreover, schools, institutions of higher education, hospitals, and commercial buildings have the potential to benefit from biogas technologies and facilities (Parawira 2009 ). Excluding South Africa, insufficient scientific literature has reported technology development of the commercial biogas system in Africa. In the Southern parts of Africa, developed technologies are the lagoon, plug low, and up-flow sludge blanket (UASB) (Mutungwazi et al. 2018 ).

Biogas production and utilization

In this section, biogas production from wastewater treatment plants (WWTP), biowaste digestion, agricultural products (largely manure and energy crops), waste stream from different industries, and landfill gas are considered. In Europe, Germany has dominated the industry by far in which its annual production is accounted for 120 TWh followed by the UK with 25 TWh and 9 TWh in France. Denmark and the Netherland's production capacity is around 4 TWh and the remaining countries share is less than 3 TWh (Bioenergy 2019a ).

In Germany, the total gross electricity and heat production from biogas is about 33 TWh/year and 18.8 TWh/year, respectively. Based on statistics revealed by the Federal Ministry for Economic Affairs and Energy of Germany, a considerable amount of the biogas was utilized for electricity production (58%) and heat production (33%), and approximately only 1% was used as a vehicle fuel (Bioenergy 2019a ).

In 2018, about 32% of entire renewable heat used in the UK was produced by anaerobic digestion technology, of which 9 TWh/year was produced by biomethane, 2 TWh/year by biogas and CHP accounted for 918 GWh/year, while 2681 GWh of electricity was generated by the sector (Association ADaB. ADBA annual report 2019. 2018 ).

In France, total electricity production from biogas was about 1.8 TWh/year at the end of 2017, simultaneously total heat generated accounted for 1.7 TWh/year, which demonstrates nearly equal portion for both heat and electricity. Regarding heat production, the agriculture sector accounts for an indispensable portion, while in electricity production, the landfill has a pivotal role with 953 GWh/year followed by agriculture with 765 GWh/year (Bioenergy 2019a ).

In Denmark, the biogas sector provides 5% of the entire energy consumption of which biogas plants contribution is 60% and the rest relies on wastewater treatment plants and landfill sites. The Danish Energy Agency states that due to several support schemes such as upgrading biogas to Natural gas, biogas employment for process purposes in the industrial sectors, etc. results in promoting biogas utilization through the country (Agency and Biogas in Denmark 2019 ). Total Danish biogas production at the end of 2018 was reported to be about 1763 GWh/year in which the agriculture sector (both centralized and farm plant types) showed the largest contribution with 1367 GWh/year. 66% of produced biogas energy (which corresponds to 1150 GWh) is used to provide electricity, followed by upgrading plants with 17% portion and heat generation with 16% Bioenergy IEAI.Denmark Country Report -2019 ( 2019 ). In the Netherlands, in 2017, two co-digestion and municipal waste plants had the largest share in production, and the final use of biogas (3034 TJ heat was produced solely with municipal waste, while co-digestion had a pivotal role in electricity production representing 1825 TJ) Bioenergy IEAI.The Netherlands Country Report -2019 ( 2019 ).

In Sweden, 48% of biogas production corresponds to co-digestion plants followed by WWTPs (37%), the remaining being produced by the other plant types such as landfills, industrial facilities, and farm-based. In terms of utilization, the upgrading or transport sector represented a considerable portion (65%) followed by heat (19%), while electricity production share was almost 3% (Bioenergy 2019b ).

In Asia, China plays a significant role with 98.4% of biogas production between non-OECD countries. Primary infrastructures such as advanced industry and socioeconomic conditions have a profound impact on biogas generation and utilization growth. Small-scale and household biogas systems have been widely developed by countries like India and Bangladesh. Various researches prove that there are plenty of resources for producing biogas in developing countries when barriers such as socioeconomic, climate conditions, and appropriate technology have been addressed accurately. Several biogas plants in the range of medium to large scale have been launched in China and India (Mittal et al. 2019 ; Jiang et al. 2011 ; Gu et al. 2016 ).

In the USA, over 2200 biogas plants are operated, among which 250 AD on farms, 1269 wastewater recovery plants employing an AD, and 66 independent plants that use food waste as feed and 652 landfill gas projects. The America Biogas Council has revealed that there is still an enormous potential for developing the biogas industry in the USA where it is possible to achieve 103 trillion kWh/year (Council 2019a ). California ranks first in biogas production potential among all the 50 states in the USA (Council 2019b ), followed by Texas (Council 2019c ).

The power generation from biogas is estimated to be 9731 million kWh and 6574 million kWh electricity for California and Texas states, respectively. In California, the manure system has the highest potential with about 900 biogas plants, while currently 38 manure plants are operated with 156 wastewater facilities in Texas. (Council 2019b , c ).

In Canada, bioenergy currently provides approximately 26.7% of Canadian entire renewable energy market, the highest share is from burning solid biomass (23.1%), followed by the liquid biofuels (2.4%), and biogas (1.2%) (Canada 2019 ). In Canada, total installed plants for biogas production are estimated to be around 150. Most production takes place in landfills with 45 plants (share of 30%), followed by the agriculture sector with 37 plants (share of 24.7%) and WWTPs with 31 plants (20.7% production portion) (Association WB.Canada Market Report. 2019 . 2019 ).

Based on the Canadian Biogas Association data, at the end of 2018, about 195 MW of electricity and 400,000 GJ of Renewable Natural Gas (RNG) were generated (Biogas and Potential. 2019 . 2019 ). Biogas is utilized for providing heat and electricity, delivering to a nearby user using a pipeline, converting into electricity and connecting to the grid, or refining to RNG based on circumstances such as the landfill site location, and the energy demand of plants. In this regard, approximately 50% of the produced biogas is converted into power, with the rest going to combined heat and power (CHP) application (about 25%), heat (only 10%) and RNG (about 4%), and electricity and RNG (about 1%) (Association WB.Canada Market Report.2019. 2019 ).

In Australia, at the end of 2017, generated electricity from biogas industry was approximately 1200 GWh, which is equivalent to almost 0.5% of the entire electricity generation of the country, while biogas potential electricity generation was estimated as 103 TWh, equal to almost 9% of Australia’s entire energy consumption (Australia and Biogas opportunities for Australia. 2019 ).

The main use of biogas in Australia is for electricity with the greatest share for landfills (53.7%), followed by biowaste and WWTPs (40% and 33.3%, respectively). Heat is used in the industrial sector with a share of 30% and afterward the WWTPs with a share of 26.2%. In CHP applications, agriculture plants have the largest portion (50%), followed by the biowaste and the WWTPs (equal share of about 20% each). Between 40–50% of the excess biogas is flared at agriculture, industries, and landfills. Twenty percent of WWTPs and biowaste are no biogas upgrading plants in Australian’s biogas industry (Bioenergy IEAI.Australia Country Report. 2019 . 2019 ).

In Africa, South Africa has the largest share of installed biogas plants with about 700 plants, while only 300 plants might have been in operation as of 2007, while it can generate 148 GWh electricity from estimated biogas potential by appropriate investment and implementation schemes (Kemausuor et al. 2018 ).

Various industrial trends in the biogas production have been introduced to improve quantitative and qualitative properties of the biogas. Yet, the accomplishments of AD intended for advanced investments will increase from the low charge of feedstock accessibility and the broad range of practical set ups of the biogas (i.e., heating, electricity power, and fuel form). The remained parts of slurry from biogas production procedure have the potentials to be improved to be used as fertilizer to enhance the sustainability. Produced biogas could be employed to generate power for integrated or isolated systems in the rural and urban regions and are deemed to be economical favorable. The employed processes of AD, modern trends accompanied by included advantages and disadvantages are also demonstrated more details and progress on the way to producing biogas in a sustainable approach. Obtained results from previous researches indicated that the present amount of biogas production confirms that regarded approaches would have main influence on the energy utilization in upcoming times. The impression contains diminished release of pollutants to the atmosphere guarantees that the global warming prevention. Nevertheless, the current trend of the biogas production varies in diverse countries, either in production or the sources (landfill, AD, sewage sludge, or thermochemical methods). The involvement of biogas to the domestic natural gas utilization varies differently, around 4% on standard values; however, it raised 12% in Germany. The major nations in the biogas production in the European Union are France, Italy, Germany, Czech, and UK. Germany stands as the European frontrunner with a biogas production of 329 PJ and a contribution of 50% of total in the EU. It’s reasonable to surmise that, based on the provided data from various researches, it has been declared that given the growing need and available technology, European Union countries, and especially Germany and Sweden, will be pioneers in the development, operation, and production of biogas in the world. Table ​ Table1 1 indicates the biogas plants, upgrading units, and their upgrading capacities in certain EU countries (Lampinen 2015 ; Backman and Rogulska 2016 ; Esmaeilion et al. 2021 ).

Biogas plant in EU selected countries and their specifications

Recent progress in biogas production

Producing biogas is a key option in the energy sector of various countries. There is a wide variety of raw materials for utilization in biogas plants. In this case, obtaining a stable state in plants is a crucial concern that influences the prices and additives. Another important issue in the biogas plants is that their products should be attractive in terms of value and efficiency (Chen et al. 2012 ). Recent progress in the field of biogas production can be divided into three categories: feedstock and additives, pretreatments, and processes.

Feedstock and additives

The organic matters are the main feedstocks in the biogas plant, which can fall into different categories. Evaluating the potential of biogas production based on organic matters from rural regions has been investigated. The highly fermentative wastes can decrease the quantity of feedstock in biogas plants (Pawlita-Posmyk and Wzorek 2018 ).

Microalgae with satisfactory features is a potential option for feedstock in biogas systems. In comparison with other biomass resources, microalgae has better efficiency, more convenient production, and higher content of lipid and polysaccharide that make it a flexible choice in biogas plants (Wu et al. 2019 ). Kaparaju et al. (Kaparaju et al. 2009 ) explored the production of biogas from sugars released from wheat straw with the aid of hydrothermal pretreatment based on the biorefinery procedure. In this case, the pretreatment process increased the gas yield by 10%.

For achieving sustainable progress, the global trend of energy production is moving to the waste-to-energy (WTE) method which has multilateral benefits. Currently, biomass resources are being employed to generate energy. All around the world, biomass satisfies around 50 exajoule of the entire energy demand annually (Steubing et al. 2010 ; Ferreira et al. 2017 ; Ahmadi et al. 2020 ).

A broad spectrum of waste types can be consumed as a feedstock in biogas units by anaerobic digestion (AD) technology. Huge amounts of lignocellulosic waste could be collected from agricultural and municipal resources. The most common types of waste and residuals that can be used in the biogas sector are animal manures and dungs, muck and slurry, domestic/municipal wastewater (sewage), mud (sludge), urban garbage or municipal solid waste (MSW), and food substances loss. Table ​ Table2 2 indicates the power generation and associated yields of biogas production by accessible resources (Waste-to-energy 2015 ; Stucki et al. 2011 ).

Comparison between different resources in terms of biogas yield and electricity generation

Considered efficiency for electricity production is 35% in CHP

To enhance the yield of biogas production, utilization of additives is an acceptable method. Specifications of these components can be varied based on their biological or chemical properties under various conditions. With the aid of these materials, desirable conditions for bacteria could be provided. However, biocenosis features are vital for achieving the ideal concentration (Demirel and Scherer 2011 ).

Using salts with Mg and Ca improves methane production efficiency with low slurry foaming (Sreekrishnan et al. 2004 ). For stabilizing pH fluctuations and reducing the contents of NH 3 and H 2 S, several types of additives have been studied (Kuttner et al. 2015 ). Furthermore, using zeolite compounds has the potential to intensify the quantity of biogas production by 15%, also the addition of CaCO 3 can improve this yield by 8%. Adding biological additives increased the production rate of biomethane and biogas by optimizing AD (Vervaeren et al. 2010 ). Using biological additives is a common way of increasing biogas production yield. Yi Zheng et al. (Zheng et al. 2014 ) stated that by adding enzymes to lignocellulosic biomass, biogas production was enhanced by 34%. Vervaeren et al. (Vervaeren et al. 2010 ) reported that by adding homo and hetero-fermentative bacteria to maize components, production yield increased by 22.5%. With the addition of fungi compounds (e.g., ceriporiopsis subvermispora ATCC 96,608) to the yard trimmings, methane production increased by 154% (Zhao 2013 ). The alternative options for biological additives are chemical compounds. Using a wide variety of chemical additives like NaOH, Ca(OH) 2 , NH 4 OH, H 3 PO 4, etc., can improve the associated biogas production yield. Chandra et al. reported the effects of using NaOH as an additive to the wheat straw. Obtained results presented that yield of methane could be improved by up to 112% (Chandra et al. 2012 ). Badshah et al. investigated the diluted H 2 SO 4 properties, added to the sugarcane bagasse, which could increase the production rate by up to 166% in comparison with pre-additive treatments (Badshah et al. 2012 ).

The impact of activator addition on the biogas quality slurry is investigated in Indonesia (Ginting 2020 ), the study started by adding new bioactivator prepared from agricultural wastes such as bananas, papayas, and pineapples waste with an additional of chicken intestines where the bacteria in the chicken intestine are effective at work. The addition of the activator resulted optimally in the work where stable gas production was achieved. The slurry at the end of the production process was a liquid fertilizer ready to use. The study showed the best concentration of the activator in the production process of both the slurry and the biogas.

Pretreatment

Predominantly, there are two wide-ranging classifications for biogas production upgradation, ex situ, and in situ techniques, while most of the methods focus on ex situ approaches. Some of the conventional ex situ treatments are adsorption, catalytic processes (e.g., biological or chemical), membrane gas permeation, desulfurization, scrubbing, and absorption. Sarker et al. ( 2018 ) overviewed the in situ biogas production upgrades.

With the help of the in situ method, the associated cost concerning cleaning techniques could be reduced and the quality of produced biogas improved in the same vein. Nevertheless, the in situ method is limited to the empirical state and prototype models. Figure  2 summarizes various types of biogas upgrading methods (Sarker et al. 2018 ; Bassani et al. 2016 ; Rachbauer et al. 2016 ; Lemmer et al. 2015 ).

Biogas improvement by ex situ and in situ techniques (Sarker et al. 2018 ; Bassani et al. 2016 ; Rachbauer et al. 2016 ; Lemmer et al. 2015 )

The pretreatment productivity influences the associated bioprocess efficiency of lignocellulose. Pretreatment techniques are intended to make AD faster, enhancing the yield of the biogas, and producing a broad range of usable substrates.

Figure  3 indicates the mentioned effects of pretreatment processes. By considering efficiency, economy, and application as objective functions, optimization of pretreatment processes is a necessitated aim. Pretreatment should be operative in eradicating the structural obstacles of associated polymers with lignocellulose (it should be noted that the cellulose and hemicellulose constituents are in this classification), through exposing these substances to microbial decay efforts, which increases the biomass degradation and consequently enhances the biogas yield (Spyridon et al. 2016 ).

Pretreatment effects on the value of anaerobic digestion ( b ) and yield of CH 4 ( c ) (Achinas et al. 2017 )

There are crucial requirements in common designs of biogas plants for increasing the rate of gas production. Recently, innovative designs of biogas plants have been introduced (e.g., Konark, Deenbandhu, and Utkal Models) (Sreekrishnan et al. 2004 ; Kalia and Singh 2004 ; Abouelenien et al. 2010 ; Prasad et al. 2017 ) in which the design parameters changed to increase productivity and effectiveness in cost factors. In these concepts, by implementing optimum measurements in regarded shapes (similar to the spiral shape), the index of gas storage volume was enhanced by 33–50%, while the related costs were reduced by 10–15%.

The hydrolysis of a high proportion of non-biodegradable compositions from MSW (which is intractable by AD) can be performed by microwaving or autoclaving (Pecorini et al. 2016 ). In another study, by applying pressure to biowaste in the pretreatment procedure, biogas yields were improved significantly (Micolucci et al. 2016 ). The most desirable condition in the pretreatment of biomass is to provide an ideal environment for breaking down the feedstock substances to the sugars that are fermentable, by increasing the accessibility for microorganisms. This process leads to eradicating the lignin endurance and declining the cellulose’s crystalline formation (Micolucci et al. 2016 ). Table ​ Table3 3 presents the merits and demerits of various pretreatment technologies.

Merits and demerits of pretreatment techniques

By implementing fast pyrolysis pretreatment, biogas production has been increased (Wang et al. 2016a ). This innovative approach in thermochemical pretreatment with the aid of a lower temperature fast pyrolysis (LTFP) to enhance the performance of the AD process has been introduced, in which corn stover was used as a primary substance.

During the pretreatment procedure, a fluidized bed pyrolysis reactor applied high-temperature gas flow at 200 °C. To improve the efficiency, different strategies in the pretreatment section were performed (e.g., characteristics analysis, assessing crystal concentration of the corn stover components). Comparing the results obtained between pre- and post-treatment, the production efficiency of methane increased by about 18%. In thermochemical pretreatment, chemical bonds in substances would be broken by implementing the thermo-physical process. Biogas production and hydrolysis of celluloses are affected by the degradation of hemicellulose and lignin (Cara et al. 2006 ). Thus steam explosion falls into this category (Bauer et al. 2014 ). In this method, biomass is subjected to high-temperature steam at 240 °C, so that after a long time, morphological and chemical transformations in biomass can occur (Biswas et al. 2011 ). Another pretreatment method to upgrade the biomass is the Torrefaction process which is applied to produce a higher amount of hydrophobic fuel with a fixed range of carbon content. The operational temperature for this process is from 200 to 300 °C in a stable environment (Mafu et al. 2016 ; Sarkar et al. 2014 ). Fast pyrolysis is an additional pretreatment that was highly used in the field of biofuel production. In this case, by reducing the temperature (around 200 °C) lignin and hemicellulose could be wrecked. Nonetheless, there is no study demonstrating an increase in biogas production (Bridgwater 2012 ; Y-m et al. 2009 ). Rodriguez et al. (Rodriguez et al. 2017 ) investigated different pretreatments for grass in biogas production sectors. The obtained results revealed that all pretreatments could increase biogas production by around 50% even though all of them suffer from high energy consumption.

The ultrasonic pretreatment process is an innovative and practical technique in the pretreatment section. This process increases the efficiency of sludge dewatering, stability of the digestion, solids solubility, and rate of biogas production. The outcome of this method is a digestate containing a low share of residual organic materials. The ultrasonication modifies the biological, chemical, and physical specifications of the sludge. Some of these variations are pathogen reduction, settling velocity improvement, and protein concentrations increase (Cella et al. 2016 ; Liu et al. 2015 ; Feng et al. 2009 ).

By applying this pretreatment, the rate of CH 4 production increased by 34% (up to 80% of energy consumption in the pretreatment unit is reachable by produced methane) (Mirmasoumi et al. 2018 ). The Lysis centrifuge consists of a method focused on centrifuge which initiates partial destruction in sludge cells. This strategy can improve biogas production by 15–26% with thickened sludge resources. This practice is suitable in pretreatment processes (for dewatering) and does not impose any extra load on the system for extra operations (Dohányos et al. 1997 ).

Biological pretreatment is an alternative for thermal and chemical pretreatment that is composed of different stages like enzymatic hydrolysis, using fungi additives and thermal phased AD (TPAD). Among named processes, TPAD has attracted attention. The benefits of this biological pretreatment are lower energy consumption and higher biogas production in comparison with other methods (Zhen et al. 2017 ; Bolzonella et al. 2012 ).

By comparing the results between thermal and autohydrolysis pretreatments, the production of biogas in the biological procedure is considerably lower than in the thermal pretreatment (26% and 45%, correspondingly). The dominant conditions of autohydrolysis pretreatment were reported to be at 55 °C for 12–24 h compared with 170 °C for half an hour for thermal pretreatment (Carvajal et al. 2013 ). In this field, the highest yield achieved in biogas production was investigated by Bolzonella et al. (Bolzonella et al. 2012 ) by applying the pretreatment at 70 °C for 2 days, with associated yield increasing by up to 145%. It is noteworthy to mention that many studies have investigated the combined pretreatment for increasing the biogas production yield (Liu et al. 2018 ; Bao et al. 2015 ; Chan et al. 2016 ; Abelleira-Pereira et al. 2015 ; Wang et al. 2014 ; Bentayeb et al. 2013 ) however, this is out of the scope of this study.

The biogas production can be categorized into two main fermentation process which are dry and wet processes. For the digestion by wet process, the overall solids concentration in the fermenter is lower than 10%. To treat solid substrates, using liquid manure for achieving pumpable slurry is necessary. On the other hand, in the dry digestion, the overall concentration of solids in the fermenter is ranging from 15 to 35%. The stability in the wet digestion processes is higher than in dry methods. In the agricultural section, wet digestion practices are more widespread (Weiland 2010 ).

The biogas production procedure includes four important phases which are hydrolysis, acidogenesis, acetogenesis, and methanogenesis as can be seen in Fig.  4 .

Diagram of the biogas production procedures by AD (Mao et al. 2015 ; Visvanathan and Abeynayaka 2012 )

For developing methane fermentation, diverse associations of bacteria are needed, which are aceticlastic and hydrogenotrophic methanogens, syntrophic acetogens, fermentative bacteria, and homoacetogens. The balanced contribution between them increases the efficiency of biogas production and the AD process (Chen et al. 2016 ). There is a specific type of AD that involves anaerobic membrane bioreactors (AnMBRs), which increases the quantity of biogas production by membrane specifications. By considering the techno-economical parameters of AnMBRs, the efficiency of biogas production has the potential to be increased dramatically (Chen et al. 2016 ). Figure  5 shows the different types of AnMBR technologies.

The biogas production processes by different types of AnMBR technologies

The methanogenic organisms have a negative instinct for sluggish growing, and also the complexities of microbial in the systems have caused difficulty in the functioning of biogas fermenters. An innovative concept of integrating the anaerobic bioprocess with membrane breakdown practice through a membrane bioreactor (MBR) allowed augmenting the biomass concentration through a bioreactor. With an anaerobic membrane bioreactor (AnMBR), high hydraulic load, and adequate mixing brought sustainability for high cell concentrations (Wang et al. 2011 ).

The AnMBRs have a special feature for providing satisfactory retention of active microorganisms. This specification leads to optimal productivity and favorable resistance against toxic substances. Furthermore, high concentrations in the final product and easy separation of biomass and products (by micro-/ultra-filtration) have been added to its benefits (Ylitervo et al. 2013 ). Obtained results revealed that methane yield in biogas production was up to 0.36 l CH 4 /g chemical oxygen demand (COD) and methane content reached 90% (Liao et al. 2010 ).

Wang et al. (Wang et al. 2011 ) discussed the developing approaches for the biogas sector in China and presented every aspect of this technology including the AnMBRs. Ylitervo et al. reviewed the MBR strategy for producing ethanol and biogas and explained the progress in MBRs (Ylitervo et al. 2013 ). Minardi et al. (Minardi et al. 2015 ) reported various applications of the membrane in biogas technologies and purification methods. Mao et al. (He et al. 2012 ) investigated the latest trends in biogas production by AD and AnMBRs. To improve the efficiency of AD, numerous investigations have been focused on various configurations (like single- or multiple-stage reactors).

The latest studies considered the breakdown of the AD method into two groups. For example, acetogenesis–methanation and hydrolysis–acidogenesis are accomplished in unconnected reactors, which can enhance the rate of the conversion process of organic matters to CH 4 , although the high prices associated with these types of systems are a critical issue (Yu et al. 2017 ).

More stability and improved efficiency are the outcomes of utilizing multiple-stage bioreactor systems. These types of systems allow for different conditions to be implemented. Obtained results from (Colussi et al. 2013 ) revealed that the two-step AD of corn requires a greater oxygen demand. Marín Pérez et al. (Pérez and Weber 2013 ) stated that the AD physical parting into two phases established the acceptance of various procedure settings for a particular bacteria type, which increased the degradation rate of organic materials. For preventing ammonia inhibition, the two-stage AD of MSW has been implemented (Yabu et al. 2011 ).

A study conducted in 2008 evaluated the one- and two-stage AD in terms of performance. Results showed that the two-stage process had an advanced yield of CH 4 production (Park et al. 2008 ). Figure  6 shows the schematic of multi-stage AD technology.

Standard diagram of a multiple-stage scheme of AD technology

A two-stage AD system has a suitable potential to process a variety of residuals with high microbiological contents. Blonskaja et al. (Blonskaja et al. 2003 ) stated that by using a two-phase AD for distillery waste, a higher rate of methane would be produced. Kim et al. (Kim et al. 2011 ) implemented a four-phase scheme for activated slurry, which allowed extraordinary digestion productivity. The latest improvements in the utilization of molecular biology implements have developed the utility of included microorganisms and the knowledge of the AD practice. Bioindicators and innovative eco-physiological considerations are the ultimate enhancements of the chemical indexes for monitoring and controlling the stability of the AD process (Lebuhn et al. 2014 ). AD process with renewable feedstock has been introduced as a forthcoming method for biogas production. The biogas chiefly consisted of CH 4 (60%) and CO 2 (35–40%). (Abdeshahian et al. 2016 ).

With the aid of the pyrolysis process, pyrolysis gas from biomass resources can be produced. Pyrolysis gas consists of carbon monoxide, hydrogen, carbon dioxide, and extra gases in minor quantities, e.g., methane and some specific components. The biomass resources are lignocellulosic biomass, MSW, lignite, and digestate. The most important advantage of pyrolysis is that the organic components (specifically the relatively dry and gradually biodegradable biomass that is not appropriate for the AD process) can be converted to pyrolysis gas (Luo et al. 2016 ). In the pyrolysis gas production, a methanation process is essential. Traditional catalytic methanation needs high pressure and temperature (230–700 °C) and a metal catalyst, which imposes high cost with low energy efficiency (Guiot et al. 2011 ). Li et al. (Li et al. 2017a ) have investigated the new approach for employing pyrolysis products as a reservoir of carbon for biogas production. In this study, the effects of different parameters on biomethanation of pyrolysis gas have been assessed.

Different strategies, i.e., hydrothermal pretreatment (HTPT), ultrasonic method, alkaline method, and a combination of them, have been used for the dewatering of biomass materials. By considering every aspect of their functions, HTPT has provided the intended benefits (e.g., hot compressed water utilization and decomposing extracellular polymeric substances) (Park et al. 2017 ; Ruiz-Hernando et al. 2015 ).

A prototype for combining hydrothermal pretreatment with pyrolysis and AD process for cogeneration of biogas and biochar has been presented (Li et al. 2018 ). In the hydrothermal pretreatment (HTPT) stage, by heating sludge at 180 °C for half an hour, the water content fell significantly (from 85 to 33%) and dewaterability improved. After that, filtration outputs were subjected to mesophilic AD without any interruption at an approximate temperature of 37 °C. An up-flow anaerobic sludge-bed reactor has been used for biogas production to be consumed in the hydrothermal pretreatment section. Concurrently, for producing heavy biochar, a rotary kiln has been utilized for filter cake pyrolysis at about 600 °C. The considered configuration included a boiler, a pressure filter, a cooling chamber, and a hydrothermal reactor. The sludge was fed into the first reactor (A) and for diluting the sludge, some water was added (20%). In the next reactor, the superheated steam raised the temperature of the sludge (190 °C). By discharging the steam to the first reactor, the pressure in the second reactor decreased (less than 0.11 MPa) and drained steam used for preheating the input sludge (Li et al. 2017b ).

Hübner et.al (Hübner and Mumme 2015 ) proposed a design for biogas production by using aqueous liquor from digestate pyrolysis. In the applied conditions, three main liquors were produced by the pyrolysis process (at 330, 430, and 530 °C) under four chemical oxygen demand (COD) concentrations (3, 6, 12, and 30 g.L −1 ). At 3 g.L −1 , 6 g.L −1 , and 12 g.L −1 a considerable increase in biogas has been observed. Besides, an important feature was that the biogas production in this process did not need any additives.

The studies based on the microbiology field are developing the concept of hydrolytic microbes and biogas production correlation. These types of investigations focused on the hydrolytic microorganisms’ involvement in biogas units, metabolism types, and their functionality in regarded processes. Azman et al. (Azman et al. 2015 ) studied the participation of anaerobic hydrolytic microbes in biogas production from lignocellulosic (by considering microbiological features). Nina Kolesáarová et al. (Kolesárová et al. 2011 ) examined the possibilities for producing biogas with biodiesel by-product as a feedstock in various phases. Yang et al. (Yang et al. 2014 ) presented a membrane gas-permeation for biogas upgrading. In this study, the authors implemented polymer membranes to upgrade biogas production. Furthermore, Miltner et al. (Miltner et al. 2017 ) reviewed innovative technologies in purification and production of biogas. Kiros Hagos et al. (Hagos et al. 2017 ) presented an anaerobic co-digestion (AcoD) process for producing biogas from various diverse biodegradable organic sources. The digestate (fermentation residue) had a high content of moisture that should be dried for increasing the nutrient concentration and decreasing the transported mass. In this case, using a solar greenhouse dryer in tandem with heat recovery from combined heat and power and a microturbine provided a logical opportunity to eradicate the undesirable moisture content. The hybrid case had the potential to reduce moisture content by up to 80% (Maurer and Müller 2019 ). Owing to the faster reaction rates and higher productivity, the thermophilic digestion method is more satisfactory than mesophilic digestion. The mesophilic digestion method leads to a low methane yield and the related biodegradability is relatively poor. On the other hand, these systems represent enhanced stability and higher concentration in bacteria distribution. Unexpected thermal fluctuations affect methanogens performance; as a result, any extreme variation in temperature is undesirable. In this case, it is better to coat the facilities of biogas plants with insulators to control the digester temperature. By building sun-facing biogas units, the effect of cold winds would be eradicated. The integrated system consisted of a solar system and a biogas plant, which provided satisfactory results in gas yield values during cold seasons (Horváth et al. 2016 ).

Therefore, it is reasonable to surmise that biogas production has been influenced by different parameters and factors, including pretreatment processes, feedstock, and additives features, and process technologies. Provided data appear to confirm the following summary of key points.

• Production of biogas is an approach for biomass treatment and can help energy generation sustainably. Proper potentials for fossil fuel replacements increased the attention to biogas upgrading and advanced pretreatment methods. The biogas pretreatment procedure has two main steps: 1. biogas cleaning methods and 2. biogas upgrading method. With the help of these strategies, the lignin layer would be broken and the biomass turns to a suitable feedstock for the digestion process, while the porosity increases simultaneously. Hereon, the biogas yield would be improved (based on the feedstock types and associated technologies, obtained rates would be different).
• There are different techniques for biogas upgrading that each one has a specific contribution based on the applied commercial technologies. Waster scrubber, chemical scrubber, membrane pressure swing adsorption, and organic physical scrubber contributed the most accounting for 35%, 21%, 20%, 17%, and 5%, respectively.
• An extensive variety of compositions has been evaluated and observed for biogas production. Crop biomasses (wheat, barley, etc.), organic wastes (MSW, agro-industry wastewaters, animal manners, etc.), crop residues (wheat straw, barley, or rice straw, etc.), and non-conventional biomass (microalgae or glycerol) fall into this category. Using a wide variety of chemical additives like NaOH, Ca(OH) 2 , NH 4 OH, H 3 PO 4, etc. can improve the biogas production yield. Using additives can improve the AD process stability and lead to up to 40% higher methane yield.

Recent research has been conducted using carbon membranes for biogas upgrading (Lie 2005 ) where the gas separation process was faster. The selected membranes were thin carbon layers with a thickness of less than 1  μ m supported on ceramic tubes with a length of 0.50 m. Permeation tests using these membranes showed that the CO 2 molecules permeate 50 times faster than CH 4 molecules. By using such membranes, a typical gas mixture consisted of 0.6 of CH 4 is enriched with CH 4 by only one step separation process to more than 0.9 at 1.20 MPa. The membranes showed excellent mechanical properties after a one-month test. The same membranes are used to separate other gases in the biogas mixture such as H 2 S gas. Such new technologies helped a lot in the biogas industrial process in terms of cost reduction and energy consumption compared to classical technologies such as scrubbing.

As mentioned in this part, process, pretreatment, and feedstock are the main influential parameters for biogas production. For obtaining the highest yield in this term, forming a proper balance between these factors has a significant impact on efficiency. Biomass comprises carbohydrate matters, proteins substances, fats, cellulose, and hemicellulose, which could be employed as raw materials for biogas production. In the existing method, co-substrates improved gas yield by increasing the organic content. Distinctive co-substrates contain organic wastes from agriculture-linked productions, food leftover, and gathered municipal wastes from houses. The composition and yield of biogas production be determined by the feedstock and co-substrate category. Although carbohydrates or proteins demonstrate quicker transformation degrees than fats, it is stated that the second one provides more biogas yield (Achinas et al. 2017 ; Braun 2007 ). To keep away from process non-fulfillments, pretreatment is essential. Employing pretreatment approaches improves the degradation of substrates and then the process productivity. Chemical, thermal, mechanical, or enzymatic procedures can be used to accelerate the decomposition method, while this doesn’t unavoidably affect an advanced yield (Putatunda et al. 2020 ; Mshandete et al. 2006 ).

Policy and framework conditions

The biogas industry expands and develops as it represents an alternative source of energy and has a direct influence on the economy. Worldwide many countries organize the market of biogas through policies and regulations. Well-prepared policies prosper the market of biogas as a renewable energy source.

For instance, by EU policies, instructions, and strategic planning, the portion of renewable energies from 2005 to 2015 increased from 9% up to 16.7%, which is predicted to rise to 20% until 2020 (Irena 2018 ).

Keeping these long-term policies with continuous revision and evaluation is a leader in the world of biogas utilization and marketing (Al Seadi et al. 2000 ; Torrijos 2016 ).

Several organizations and governments such as the European Biogas Association (EBA) and the European Parliament and Council legislated regulations in this regard (Xue et al. 2020 ). The role of such organizations is to prepare new relevant policies for upcoming issues and update existing policies to satisfy the market needs and fluctuations and to harmonize the environment and investment. For example, in the UK there are 118 renewable energy policies compared to 7, 28, and 32 in Denmark, Italy, and German, respectively. This is a common framework, and it is used to write their national policies for organizing renewable energy in Europe. Despite the presence of common European directives such as (2009/28/EC) that considered the biogas production from of agronomic deposits and organic trashes and its application in producing power and heat, several EU nations established their energy markets and biomass sources. These countries issued policies to satisfy their own needs and priorities which known as the National Renewable Energy Action Plan.

China has more than 25 energy policies to manage renewable energy. These policies support the renewable market. Biogas was among these renewable energy sources that benefit from such policies and regulations to develop. The Chinese government governmental parties such as the State Council (SC), the Party Central Committee (PCC), and the Ministry of Agriculture and Rural Affairs (MARA) of China participated extensively in developing policies, regulations, and instruction relevant to the progress of biogas (Gu et al. 2016 ; Hua et al. 2016 ; Wang et al. 2016b ). There is a policy about the development necessities of biogas in rural zones that must be updated every year since 2004 it is called Central Document No. 1 (Ndrc 2017 ).

To point out some Chinese policies, the policy of Measures for the Administration of Rural Biogas Construction National Debt Projects (Trial) was developed in 2003 by its Ministry of Ecology and Environment (MEE) and Rural Biogas Project Construction Fund Management Measures was developed by both MARA and Ministry of Finance (MF) of China in 2007.

Recently in 2019, there has been two policies that resulted in numerous ideas to follow the significant growth of farming and rustic zones, the experimental work program establishing waste-free cities by the state council (SC), and the friendly waste of rustic facilities of biogas production by (MARA). All of these policies regarding the handling, management, utilization, and safe disposal technologies are developed by many authorities in china to avoid the work duplication that might retard the international investment and privatization programs. Clear organization between different authorities helps in generating national priorities and smooth management this includes agricultural activities, finance, trade, and scientific research. For example, introducing a fixed premium subsidy enabled the development of biogas and green gas projects, where a finite budget for a subsidy was determined first. Another country introduced what so-called bioeconomy, especially for such projects. Finally, the harmonized sales tax (HST) is paid on purchases/expenses related to commercial construction and operation of biogas facilities which is also called (input tax credits).

Policy framework

It is important in any policy development to have certain targets to be achieved. These targets are dependent on national priorities, so it is changed from one country to another despite having common targets. Examples of targets can be achieving sustainable development for environment elements, communicate clear standards and regulations for wastes management, environmental laws to regulate the relevant processes and etc. To achieve these targets, action plans revised on a yearly basis to evaluate and update the current regulations for future use.

In general, there are five phases to develop a certain biogas policy, which are:

Phase I creating one regulatory body to coordinate the efforts of all stakeholders who can affect/ be affected by the activities of biogas management, by selecting one focal point to help in decision making and future development. This focal point can be from government or from non-governmental organizations (NGOs). This will unify the efforts to get national priorities and plans.

Phase II developing comprehensive and clear instructions and requirements for biogas production that includes management of raw materials, biogas, safe disposal of biogas wastes, preparing environmental impact assessment (EIA) for current and future facilities, applying the waste hierarchy which is referred to as 5Rs (responsibility, reduce, reuse, recycle, recover), issuing the license and permit to work, and applying the periodic environmental audit.

It is also important to apply the proximity principle for the newly constructed biogas plants and make sure to have a centralized biogas plant where all raw materials from different sources can reach it. The idea behind the one huge centralized biogas plant is to make the audit, monitoring, waste collection, transportation, packaging, labeling, storing and/or and safe disposal as easy as possible and make sure that the best and correct disposal method is applied, for example, in Denmark the Danish Government introduced a total ban on landfilling organic or combustible wastes in 1997 (Al seadi T. 2017 ).

Phase III providing incentives and subsidizing to encourage the facilities to produce biogas and increase its contribution to the economy which is referred to as the green economy and to encourage the partnership with the private sector. Such a program will help the facilities that deal with biogas in rehabilitation activities and waste management. Such incentives and subsidy include tax-free period, free consultation, reduced tariff for raw materials used in the manufacturing procedure, and decrease in the payback period increasing the return on investment of the coming projects which in turn will help in mitigation the biogas sustainability challenges. Contingency plans for unexpected challenges must be considered. The sectors of energy and renewable energy are exposed to many parameters that can affect the energy market such as wars, natural disasters, and nowadays the pandemic of COVID-19 where the prices of oil are dropped drastically (Hübner and Mumme 2015 ) and negative effects are imposed on the industry.

Phase IV providing scientific support to the projects of biogas and waste-to-energy plants. This is necessary to use the best environmental practices (BEP) and the best available techniques (BAT). This can be achieved by technology transfer and scientific research, where each plant must have a research and development (RD) department. Ministry of higher education or any relevant authority in the countries with cooperation with industry can provide funds to universities and plants for more research to utilize the waste in producing energy.

Phase VI participating in international conventions and agreements. Each country must participate in international activities and international conventions relevant to waste management and W-t- E initiatives such as Basel conventions (Basel, 2020) that regulate the transboundary movements of hazardous wastes. This participation is important to make use from the experience of each other and to get the consultation from international experts and to get fund for environmental projects from international agencies such as the German Technical Cooperation Agency (GTZ), Japan International Cooperation Agency (JICA), and United States Agency for International Development (USAID),

Phase VII training and awareness programs, where the concerned parties of W-to-E activities prepare training programs for its staff in the fields of waste management, national and international laws, environmental auditing, risk assessment/management, inspection and licensing. Also, the awareness program for the public is important to educate the people in cleaner production and relevant environmental issues. The role of universities is also important to introduce courses for undergraduate and postgraduate students to raise awareness and support scientific research.

Conclusion and recommendation

With the new applications of biogas, the worldwide biogas industry has increased by more than 90% between the years 2010 and 2018, while further growth is still expected. However, the biogas industry varies significantly in different locations over all the world. Different countries have developed several types of biogas systems which are mainly dependent on different environments as well as on energy demand and supply chain. In this study, the production processes and specific applications of biogas in recent years were reviewed and discussed. In the lack of oxygen, the disintegration of organic material produces biogas that mostly consists of carbon dioxide and methane. In recent years, the exploitation of biogas and the expansion of its potential applications have gained popularity due to factors like climate change, reasonable energy prices, and an increase in distributed generation. Biogas also traditionally known as an off-grid energy resource and can be used in various applications consisting of electricity production and CHP systems. The following key points are summarized from the study:

• It is envisioned that the extraction of intrinsic chemical energy of biomass with an efficient AD process can be achieved with proper microbial resource management. Further, advanced monitoring and control of the AD process are needed for the hour for decision making to improve the conversion productivity of the procedure by decreasing the loss of potential methane production due to imbalances of biomass charging rate.
• A sustainable circular economy can be created through biomass utilization by recycling organic residues including nutrients in order to bring it back to the society as energy and fuel.
• Upgradation of the existing technology for efficient conversion of biomass-based organic residues to biomethane and its utilization as a substitute natural gas or vehicle fuel is the trending research scope.
• Hydrogen production using a biogas reforming system with high efficiency is one of the recent applications of biogas. The progress in the application of hydrogen as a clean fuel especially for vehicles is very promising.
• Another cutting-edge application of biogas is fuel cells. Recent advances in fuel cells resulting in low emissions (CO 2 , NO x ) and high efficiency make them suitable for power generation and transportation purposes.
• Even though the conversion of biomass to biogas through AD has already become a touchable reality in many countries, high financial risks linked to its establishment seek higher financial incentives from the policymakers for sustainable shifting of existing technologies.

Failure of the extraction/utilization of renewable energy sources does not sanction the researchers to explore further, but to transfer any sustainable technology from laboratory to the market seeks ground-breaking effort of the researchers and incentives from the policymakers to handle wisely the transition period of partial/full replacement(s)/modification(s) of the existing technologies/ infrastructures, and social acceptance of the simplified—and perhaps definitive—application of the renewables.

Acknowledgements

No financial support exists in this paper.

Abbreviations

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Fact Sheet | Biogas: Converting Waste to Energy

October 3, 2017

The United States produces more than 70 million tons of organic waste each year. While source reduction and feeding the hungry are necessary priorities for reducing needless food waste, organic wastes are numerous and extend to non-edible sources, including livestock manure, agriculture wastes, waste water, and inedible food wastes. When these wastes are improperly managed, they pose a significant risk to the environment and public health. Pathogens, chemicals, antibiotics, and nutrients present in wastes can contaminate surface and ground waters through runoff or by leaching into soils. Excess nutrients cause algal blooms, harm wildlife, and infect drinking water. Drinking water with high levels of nitrates is linked to hyperthyroidism and blue-baby syndrome. Municipal water utilities treat drinking water to remove nitrates, but it is costly to do so.

Organic wastes also generate large amounts of methane as they decompose. Methane is a powerful greenhouse gas that traps heat in the atmosphere more efficiently than carbon dioxide. Given equal amounts of methane and carbon dioxide, methane will absorb 86 times more heat in 20 years than carbon dioxide. To reduce greenhouse gas emissions and the risk of pollution to waterways, organic waste can be removed and used to produce biogas, a renewable source of energy. When displacing fossil fuels, biogas creates further emission reductions, sometimes resulting in carbon negative systems. Despite the numerous potential benefits of organic waste utilization, including environmental protection, investment and job creation, the United States currently only has 2,200 operating biogas systems, representing less than 20 percent of the total potential.

Introduction  

What is biogas.

Biogas is produced after organic materials (plant and animal products) are broken down by bacteria in an oxygen-free environment, a process called anaerobic digestion. Biogas systems use anaerobic digestion to recycle these organic materials, turning them into biogas, which contains both energy (gas), and valuable soil products (liquids and solids).

Anaerobic digestion already occurs in nature, landfills, and some livestock manure management systems, but can be optimized, controlled, and contained using an anaerobic digester. Biogas contains roughly 50-70 percent methane, 30-40 percent carbon dioxide, and trace amounts of other gases. The liquid and solid digested material, called digestate, is frequently used as a soil amendment.

Some organic wastes are more difficult to break down in a digester than others. Food waste, fats, oils, and greases are the easiest organic wastes to break down, while livestock waste tends to be the most difficult. Mixing multiple wastes in the same digester, referred to as co-digestion, can help increase biogas yields. Warmer digesters, typically kept between 30 to 38 degrees Celsius (86-100 Fahrenheit), can also help wastes break down more quickly.

After biogas is captured, it can produce heat and electricity for use in engines, microturbines, and fuel cells. Biogas can also be upgraded into biomethane, also called renewable natural gas or RNG, and injected into natural gas pipelines or used as a vehicle fuel.

The United States currently has 2,200 operating biogas systems across all 50 states, and has the potential to add over 13,500 new systems.

The Benefits of Biogas

Stored biogas can provide a clean, renewable, and reliable source of baseload power in place of coal or natural gas. Baseload power is consistently produced to meet minimum power demands; renewable baseload power can complement more intermittent renewables. Similar to natural gas, biogas can also be used as a source of peak power that can be rapidly ramped up. Using stored biogas limits the amount of methane released into the atmosphere and reduces dependence on fossil fuels. The reduction of methane emissions derived from tapping all the potential biogas in the United States would be equal to the annual emissions of 800,000 to 11 million passenger vehicles. Based on a waste-to-wheels assessment, compressed natural gas derived from biogas reduces greenhouse gas emissions by up to 91 percent relative to petroleum gasoline.

In addition to climate benefits, anaerobic digestion can lower costs associated with waste remediation as well as benefit local economies. Building the 13,500 potential biogas systems in the United States could add over 335,000 temporary construction jobs and 23,000 permanent jobs. Anaerobic digestion also reduces odors, pathogens, and the risk of water pollution from livestock waste. Digestate, the material remaining after the digestion process, can be used or sold as fertilizer, reducing the need for chemical fertilizers. Digestate also can provide additional revenue when sold as livestock bedding or soil amendments.

Biogas Feedstocks  

Around 30 percent of the global food supply is lost or wasted each year. In 2010 alone, the United States produced roughly 133 billion pounds (66.5 million tons) of food waste, primarily from the residential and commercial food sectors. To address this waste, EPA’s Food Recovery Hierarchy prioritizes source reduction first, then using extra food to address hunger; animal feed or energy production are a lower priority. Food should be sent to landfills as a last resort. Unfortunately, food waste makes up 21 percent of U.S. landfills, with only 5 percent of food waste being recycled into soil improver or fertilizer. Most of this waste is sent to landfills, where it produces methane as it breaks down. While landfills may capture the resultant biogas, landfilling organic wastes provides no opportunity to recycle the nutrients from the source organic material. In 2015, the EPA and USDA set goals to reduce the amount of food waste sent to landfills by 50 percent by 2030. But even if this goal is met, there will be excess food that will need to be recycled. The energy potential is significant. As just one example, with 100 tons of food waste per day, anaerobic digestion can generate enough energy to power 800 to 1,400 homes each year. Fat, oil, and grease collected from the food service industry can also be added to an anaerobic digester to increase biogas production.

Landfill Gas

Landfills are the third largest source of human-related methane emissions in the United States. Landfills contain the same anaerobic bacteria present in a digester that break down organic materials to produce biogas, in this case landfill gas (LFG). Instead of allowing LFG to escape into the atmosphere, it can be collected and used as energy. Currently, LFG projects throughout the United States generate about 17 billion kilowatt-hours of electricity and deliver 98 billion cubic feet of LFG to natural gas pipelines or directly to end-users each year. For reference, the average U.S. home in 2015 used about 10,812 kilowatt-hours of electricity per year.

Livestock Waste

A 1,000-pound dairy cow produces an average of 80 pounds of manure each day. This manure is often stored in holding tanks before being applied to fields. Not only does the manure produce methane as it decomposes, it may contribute to excess nutrients in waterways. In 2015, livestock manure management contributed about 10 percent of all methane emissions in the United States, yet only 3 percent of livestock waste is recycled by anaerobic digesters. When livestock manure is used to produce biogas, anaerobic digestion can reduce greenhouse gas emissions, reduce odors, and reduce up to 99 percent of manure pathogens. The EPA estimates there is the potential for 8,241 livestock biogas systems, which could together generate over 13 million megawatt-hours of energy each year.

Wastewater Treatment

Many wastewater treatment plants (WWTP) already have on-site anaerobic digesters to treat sewage sludge, the solids separated during the treatment process. However, many WWTP do not have the equipment to use the biogas they produce, and flare it instead. Of the 1,269 wastewater treatment plants using an anaerobic digester, only around 860 use their biogas. If all the facilities that currently use anaerobic digestion—treating over 5 million gallons each day—were to install an energy recovery facility, the United States could reduce annual carbon dioxide emissions by 2.3 million metric tons—equal to the annual emissions from 430,000 passenger vehicles.

Crop Residues

Crop residues can include stalks, straw, and plant trimmings. Some residues are left on the field to retain soil organic content and moisture as well as prevent erosion. However, higher crop yields have increased amounts of residues and removing a portion of these can be sustainable. Sustainable harvest rates vary depending on the crop grown, soil type, and climate factors. Taking into account sustainable harvest rates, the U.S. Department of Energy estimates there are currently around 104 million tons of crop residues available at a price of $60 per dry ton. Crop residues are usually co-digested with other organic waste because their high lignin content makes them difficult to break down.

Biogas End Uses  

Raw biogas and digestate.

With little to no processing, biogas can be burned on-site to heat buildings and power boilers or even the digester itself. Biogas can be used for combined heat and power (CHP) operations, or biogas can simply be turned into electricity using a combustion engine, fuel cell, or gas turbine, with the resulting electricity being used on-site or sold onto the electric grid.

Digestate is the nutrient-rich solid or liquid material remaining after the digestion process; it contains all the recycled nutrients that were present in the original organic material but in a form more readily available for plants and soil building. The composition and nutrient content of the digestate will depend on the feedstock added to the digester. Liquid digestate can be easily spray-applied to farms as fertilizer, reducing the need to purchase synthetic fertilizers. Solid digestate can be used as livestock bedding or composted with minimal processing. Recently, the biogas industry has taken steps to create a digestate certification program, to assure safety and quality control of digestate.

Renewable Natural Gas

Renewable natural gas (RNG), or biomethane, is biogas that has been refined to remove carbon dioxide, water vapor, and other trace gases so that it meets natural gas industry standards. RNG can be injected into the existing natural gas grid (including pipelines) and used interchangeably with conventional natural gas. Natural gas (conventional and renewable) provides 26 percent of U.S. electricity, and 40 percent of natural gas is used to produce electricity. The remainder of natural gas is used for commercial purposes (heating and cooking) and for industrial ones. RNG has the potential to replace up to 10 percent of the natural gas used in the United States.

Compressed Natural Gas and Liquefied Natural Gas

Like conventional natural gas, RNG can be used as a vehicle fuel after it is converted to compressed natural gas (CNG) or liquefied natural gas (LNG). The fuel economy of CNG-powered vehicles is comparable to that of conventional gasoline vehicles and can be used in light- to heavy-duty vehicles. LNG is not as widely used as CNG because it is expensive to both produce and store, though its higher density makes LNG a better fuel for heavy-duty vehicles that travel long distances. To make the most of investments in fueling infrastructure, CNG and LNG are best suited for fleet vehicles that return to a base for refueling. The National Renewable Energy Laboratory estimates RNG could replace five percent of the natural gas used to produce electricity and 56 percent of the natural gas used to produce vehicle fuel.

Federal Policies Supporting the Biogas Industry  

The renewable fuel standard.

The Renewable Fuel Standard (RFS) was created by Congress as part of the 2005 Energy Policy Act. The RFS requires the blending of renewable fuels into the U.S. transportation fuel supply. Currently about 10 percent of the gasoline supply is provided by renewable fuel, primarily ethanol. The RFS sets fuel volumes for a variety of fuel categories: biomass-based diesel, advanced biofuel, cellulosic biofuel, and renewable fuel as a whole. Each category has a required minimum reduction in greenhouse gases.

EPA approved biogas as a qualifying cellulosic feedstock under the RFS in 2014. Cellulosic biofuels must be 60 percent less greenhouse gas-intensive than gasoline. Currently, most of the cellulosic fuel volumes are being met through the use of RNG as a vehicle fuel. Compliance with the RFS is tracked through renewable identification numbers (RINs) that can be traded, and RINs for cellulosic biofuels can earn RNG producers $40/MMBtu (as of September 2017). According to biogas producers, the RFS has become an important driver of investment in the industry.

As part of the approval of biogas, the EPA updated the RFS to allow biogas-derived electricity used as vehicle fuel to qualify for RINs, or “e-RINs.” However, as of 2017, the EPA has not approved any producer requests to start generating e-RINs, despite biogas production already exceeding current transportation electricity demand.

The Farm Bill

Programs under the Farm Bill’s Energy Title (IX) have been crucial for growth in the biogas industry. Under the 2014 Farm Bill, the USDA’s Bioenergy Program for Advanced Biofuels provides payments to producers to promote the production of advanced biofuels refined from sources other than corn starch. The program currently receives $15 million per year in mandatory funding with $20 million available per year in discretionary funding through 2018.

The Rural Energy for America Program (REAP) provides grants and loan guarantees to agricultural producers and rural small businesses to promote renewable energy production and energy efficiency improvements. The program has mandatory funding of $50 million per year through 2018, and $100 million available in discretionary funds.

The Biomass Research and Development Initiative is a joint program between the USDA and DOE. With $3 million in mandatory funding through fiscal year 2017 and $20 million in discretionary funding through fiscal year 2018, the Biomass Research and Development Board awards grants, contracts, and financial assistance to projects that stimulate research and development of biofuels and bio-based products. However, these programs have consistently seen reductions in funding through the appropriations process.

Other Agency Programs

AgSTAR is a joint program between the EPA, USDA, and DOE. The program promotes the use of anaerobic digesters on livestock farms to reduce methane emissions from animal waste. The AgSTAR program supports the planning and implementation of anaerobic digester projects, and includes state and non-governmental partners.

The EPA’s Landfill Methane Outreach Program (LMOP) encourages the waste industry to recover and use biogas generated from organic waste in landfills. LMOP forms partnerships with communities, utilities, landfill owners, and other stakeholders to provide technical assistance and seek financing for landfill biogas projects.

Conclusion  

Biogas systems turn the cost of waste management into a revenue opportunity for America’s farms, dairies, and industries. Converting waste into electricity, heat, or vehicle fuel provides a renewable source of energy that can reduce dependence on foreign oil imports, reduce greenhouse gas emissions, improve environmental quality, and increase local jobs. Biogas systems also provide an opportunity to recycle nutrients in the food supply, reducing the need for both petrochemical and mined fertilizers.

Biogas systems are a waste management solution that solve multiple problems and create multiple benefits, including revenue streams. The United States currently has the potential to add 13,500 new biogas systems, providing over 335,000 construction jobs and 23,000 permanent jobs. However, to reach its full potential, the industry needs consistent policy support. Reliable funding of Farm Bill energy title programs and a strong Renewable Fuel Standard encourage investment and innovation in the biogas industry. If the United States intends to diversify its fuel supply and take action against climate change, it should strongly consider the many benefits of biogas.

Author: Sara Tanigawa

Editor: Jessie Stolark

Two decades of studies suggest health benefits associated with plant-based diets

But researchers caution against broad diet recommendations until remaining knowledge gaps are filled.

Vegetarian and vegan diets are generally associated with better status on various medical factors linked to cardiovascular health and cancer risk, as well as lower risk of cardiovascular diseases, cancer, and death, according to a new review of 49 previously published papers. Angelo Capodici and colleagues present these findings in the open-access journal PLOS ONE on May 15, 2024.

Prior studies have linked certain diets with increased risk of cardiovascular disease and cancer. A diet that is poor in plant products and rich in meat, refined grains, sugar, and salt is associated with higher risk of death. Reducing consumption of animal-based products in favor of plant-based products has been suggested to lower the risk of cardiovascular disease and cancer. However, the overall benefits of such diets remain unclear.

To deepen understanding of the potential benefits of plant-based diets, Capodici and colleagues reviewed 48 papers published between January 2000 and June 2023 that themselves compiled evidence from multiple prior studies. Following an "umbrella" review approach, they extracted and analyzed data from the 48 papers on links between plant-based diets, cardiovascular health, and cancer risk.

Their analysis showed that, overall, vegetarian and vegan diets have a robust statistical association with better health status on a number of risk factors associated with cardiometabolic diseases, cancer, and mortality, such as blood pressure, management of blood sugar, and body mass index. Such diets are associated with reduced risk of ischemic heart disease, gastrointestinal and prostate cancer, and death from cardiovascular disease.

However, among pregnant women specifically, those with vegetarian diets faced no difference in their risk of gestational diabetes and hypertension compared to those on non-plant-based diets.

Overall, these findings suggest that plant-based diets are associated with significant health benefits. However, the researchers note, the statistical strength of this association is significantly limited by the many differences between past studies in terms of the specific diet regimens followed, patient demographics, study duration, and other factors. Moreover, some plant-based diets may introduce vitamin and mineral deficiencies for some people. Thus, the researchers caution against large-scale recommendation of plant-based diets until more research is completed.

The authors add: "Our study evaluates the different impacts of animal-free diets for cardiovascular health and cancer risk showing how a vegetarian diet can be beneficial to human health and be one of the effective preventive strategies for the two most impactful chronic diseases on human health in the 21st century."

• Diet and Weight Loss
• Diseases and Conditions
• Colon Cancer
• Endangered Plants
• Veterinary Medicine
• Colorectal cancer
• Ovarian cancer
• Polyphenol antioxidant
• Stomach cancer
• Cervical cancer
• HPV vaccine
• Breast cancer

Story Source:

Materials provided by PLOS . Note: Content may be edited for style and length.

Journal Reference :

• Angelo Capodici, Gabriele Mocciaro, Davide Gori, Matthew J. Landry, Alice Masini, Francesco Sanmarchi, Matteo Fiore, Angela Andrea Coa, Gisele Castagna, Christopher D. Gardner, Federica Guaraldi. Cardiovascular health and cancer risk associated with plant based diets: An umbrella review . PLOS ONE , 2024; 19 (5): e0300711 DOI: 10.1371/journal.pone.0300711

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Breaking news, these diets are best for lowering risk of diseases and cancer: study.

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Turns out, vegan and vegetarian diets are un-beet-able when it comes to lowering the risk of cardiovascular diseases and cancer, new research has found .

Dr. Angelo Capodici, of the University of Bologna in Italy, and his colleagues reviewed 48 papers published between January 2000 and June 2023 that investigated the link between plant-based diets, cardiovascular health and cancer risk. They found “significant” health benefits in plant-based diets.

“Our umbrella review seems consistent with other primary evidence that links the consumption of red processed meats to an increased risk of cancers of the gastrointestinal tract,” Capodici and his team wrote in their findings, published Wednesday in PLOS One .

The World Health Organization’s International Agency for Research on Cancer classified processed meat as “carcinogenic to humans” in 2015 ,  noting that there is  “sufficient evidence from epidemiological studies that eating processed meat causes colorectal cancer.” IARC also declared red meat as “probably carcinogenic to humans.”

Nevertheless, Capodici warned that “caution should be paid” before making a large-scale recommendation for plant-based diets because of limitations to the studies and potential vitamin and mineral deficiencies associated with these eating plans.

The pros of eating a diet rich in fruits, vegetables, nuts, seeds, oils, whole grains, legumes and beans have long been espoused.

Capodici and crew said lower blood pressure, better blood sugar management and a healthier body mass index are some outcomes of vegetarian and vegan diets.

They did point out that people who tend to follow these diets are “more prone to engage in healthy lifestyles,” such as regular exercise, avoidance of sugar-sweetened beverages and abstinence from alcohol and tobacco, which also reduces the risk of heart disease.

They also noted that pregnant women who adopted vegetarian diets did not lower their risk of developing gestational diabetes and hypertension compared to women who ate meat.

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And vegans risk developing anemia due to a lack of vitamin B12, an essential nutrient naturally found in animal products. Vegans are encouraged to eat grains fortified with vitamin B12 or take a daily supplement .

Capodici’s team advised that more research is needed into the effects of vegetarian and vegan diets — they say the studies they analyzed differed in dietary patterns, sample size and participant demographics, among other factors.

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